feat: 添加 Gemini 支持和 AI 参考答案生成功能

主要功能：
- 🎯 新增 Google Gemini AI 提供商支持
  - 原生 PDF 理解能力（最多1000页）
  - 完整保留图片、表格、公式等内容
  - 支持自定义 Base URL（用于代理/中转服务）

- 🤖 实现 AI 参考答案自动生成
  - 当题目缺少答案时自动调用 AI 生成参考答案
  - 支持单选、多选、判断、简答等所有题型
  - 答案标记为"AI参考答案："便于识别

- 🔧 优化文档解析功能
  - 改进中文 Prompt 提高识别准确度
  - 自动修复 JSON 中的控制字符（换行符等）
  - 智能题目类型验证和自动转换（proof→short等）
  - 增加超时时间和重试机制

- 🎨 完善管理后台配置界面
  - 新增 Gemini 配置区域
  - 突出显示 PDF 原生支持特性
  - 为其他提供商添加"仅文本"警告
  - 支持 Gemini Base URL 自定义

技术改进：
- 添加 google-genai 依赖
- 实现异步 API 调用适配
- 完善错误处理和日志输出
- 统一配置管理和数据库存储

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/pages/AdminSettings.jsx

@@ -2,26 +2,29 @@
  * Admin Settings Page - Enhanced with API Configuration
  */
 import React, { useState, useEffect } from 'react'
+import { useNavigate } from 'react-router-dom'
 import { adminAPI } from '../api/client'
 import { useAuth } from '../context/AuthContext'
-import { Settings, Save, Loader, Key, Link as LinkIcon, Eye, EyeOff } from 'lucide-react'
+import { Settings, Save, Loader, Key, Link as LinkIcon, Eye, EyeOff, ArrowLeft } from 'lucide-react'
 import toast from 'react-hot-toast'
 
 export const AdminSettings = () => {
   const { user } = useAuth()
+  const navigate = useNavigate()
   const [loading, setLoading] = useState(true)
   const [saving, setSaving] = useState(false)
   const [showApiKeys, setShowApiKeys] = useState({
     openai: false,
     anthropic: false,
-    qwen: false
+    qwen: false,
+    gemini: false
   })
 
   const [config, setConfig] = useState({
     allow_registration: true,
     max_upload_size_mb: 10,
     max_daily_uploads: 20,
-    ai_provider: 'openai',
+    ai_provider: 'gemini',
     // OpenAI
     openai_api_key: '',
     openai_base_url: 'https://api.openai.com/v1',
@@ -32,7 +35,11 @@ export const AdminSettings = () => {
     // Qwen
     qwen_api_key: '',
     qwen_base_url: 'https://dashscope.aliyuncs.com/compatible-mode/v1',
-    qwen_model: 'qwen-plus'
+    qwen_model: 'qwen-plus',
+    // Gemini
+    gemini_api_key: '',
+    gemini_base_url: '',
+    gemini_model: 'gemini-2.0-flash-exp'
   })
 
   useEffect(() => {
@@ -114,11 +121,20 @@ export const AdminSettings = () => {
       {/* Header */}
       <div className="bg-white shadow">
         <div className="max-w-5xl mx-auto px-4 py-6">
-          <div className="flex items-center gap-3">
-            <Settings className="h-8 w-8 text-primary-600" />
-            <div>
-              <h1 className="text-2xl font-bold text-gray-900">系统设置</h1>
-              <p className="text-gray-600">管理员：{user?.username}</p>
+          <div className="flex items-center justify-between">
+            <div className="flex items-center gap-3">
+              <button
+                onClick={() => navigate(-1)}
+                className="p-2 hover:bg-gray-100 rounded-lg transition-colors"
+                title="返回"
+              >
+                <ArrowLeft className="h-6 w-6 text-gray-600" />
+              </button>
+              <Settings className="h-8 w-8 text-primary-600" />
+              <div>
+                <h1 className="text-2xl font-bold text-gray-900">系统设置</h1>
+                <p className="text-gray-600">管理员：{user?.username}</p>
+              </div>
             </div>
           </div>
         </div>
@@ -189,12 +205,13 @@ export const AdminSettings = () => {
               onChange={(e) => handleChange('ai_provider', e.target.value)}
               className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent"
             >
+              <option value="gemini">Google Gemini (推荐)</option>
               <option value="openai">OpenAI (GPT)</option>
               <option value="anthropic">Anthropic (Claude)</option>
               <option value="qwen">Qwen (通义千问)</option>
             </select>
             <p className="text-sm text-gray-500 mt-1">
-              选择后在下方配置对应的 API 密钥
+              选择后在下方配置对应的 API 密钥。Gemini 支持原生 PDF 解析
             </p>
           </div>
         </div>
@@ -209,6 +226,13 @@ export const AdminSettings = () => {
             )}
           </div>
 
+          {/* Text-only warning */}
+          <div className="bg-amber-50 border border-amber-200 rounded-lg p-3">
+            <p className="text-sm text-amber-800">
+              ⚠️ OpenAI 仅支持文本解析，不支持 PDF 原生理解。PDF 文件将通过文本提取处理，可能丢失格式和图片信息。
+            </p>
+          </div>
+
           {/* API Key */}
           <div>
             <label className="block font-medium text-gray-900 mb-2">
@@ -258,16 +282,21 @@ export const AdminSettings = () => {
             <label className="block font-medium text-gray-900 mb-2">
               模型
             </label>
-            <select
+            <input
+              type="text"
+              list="openai-models"
               value={config.openai_model}
               onChange={(e) => handleChange('openai_model', e.target.value)}
-              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent"
-            >
+              placeholder="gpt-4o-mini"
+              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+            />
+            <datalist id="openai-models">
               <option value="gpt-4o">gpt-4o (最强)</option>
               <option value="gpt-4o-mini">gpt-4o-mini (推荐)</option>
               <option value="gpt-4-turbo">gpt-4-turbo</option>
               <option value="gpt-3.5-turbo">gpt-3.5-turbo</option>
-            </select>
+            </datalist>
+            <p className="text-xs text-gray-500 mt-1">可输入自定义模型名称，或从建议中选择</p>
           </div>
         </div>
 
@@ -281,6 +310,13 @@ export const AdminSettings = () => {
             )}
           </div>
 
+          {/* Text-only warning */}
+          <div className="bg-amber-50 border border-amber-200 rounded-lg p-3">
+            <p className="text-sm text-amber-800">
+              ⚠️ Anthropic 仅支持文本解析，不支持 PDF 原生理解。PDF 文件将通过文本提取处理，可能丢失格式和图片信息。
+            </p>
+          </div>
+
           {/* API Key */}
           <div>
             <label className="block font-medium text-gray-900 mb-2">
@@ -329,15 +365,21 @@ export const AdminSettings = () => {
             <label className="block font-medium text-gray-900 mb-2">
               模型
             </label>
-            <select
+            <input
+              type="text"
+              list="anthropic-models"
               value={config.anthropic_model}
               onChange={(e) => handleChange('anthropic_model', e.target.value)}
-              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent"
-            >
+              placeholder="claude-3-haiku-20240307"
+              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+            />
+            <datalist id="anthropic-models">
               <option value="claude-3-5-sonnet-20241022">claude-3-5-sonnet (最强)</option>
               <option value="claude-3-haiku-20240307">claude-3-haiku (推荐)</option>
               <option value="claude-3-opus-20240229">claude-3-opus</option>
-            </select>
+              <option value="claude-3-sonnet-20240229">claude-3-sonnet</option>
+            </datalist>
+            <p className="text-xs text-gray-500 mt-1">可输入自定义模型名称，或从建议中选择</p>
           </div>
         </div>
 
@@ -351,6 +393,13 @@ export const AdminSettings = () => {
             )}
           </div>
 
+          {/* Text-only warning */}
+          <div className="bg-amber-50 border border-amber-200 rounded-lg p-3">
+            <p className="text-sm text-amber-800">
+              ⚠️ 通义千问 仅支持文本解析，不支持 PDF 原生理解。PDF 文件将通过文本提取处理，可能丢失格式和图片信息。
+            </p>
+          </div>
+
           {/* API Key */}
           <div>
             <label className="block font-medium text-gray-900 mb-2">
@@ -400,15 +449,105 @@ export const AdminSettings = () => {
             <label className="block font-medium text-gray-900 mb-2">
               模型
             </label>
-            <select
+            <input
+              type="text"
+              list="qwen-models"
               value={config.qwen_model}
               onChange={(e) => handleChange('qwen_model', e.target.value)}
-              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent"
-            >
+              placeholder="qwen-plus"
+              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+            />
+            <datalist id="qwen-models">
               <option value="qwen-max">qwen-max (最强)</option>
               <option value="qwen-plus">qwen-plus (推荐)</option>
-              <option value="qwen-turbo">qwen-turbo</option>
-            </select>
+              <option value="qwen-turbo">qwen-turbo (快速)</option>
+              <option value="qwen-long">qwen-long (长文本)</option>
+            </datalist>
+            <p className="text-xs text-gray-500 mt-1">可输入自定义模型名称，或从建议中选择</p>
+          </div>
+        </div>
+
+        {/* Gemini Configuration */}
+        <div className={`bg-white rounded-xl shadow-md p-6 space-y-4 ${config.ai_provider === 'gemini' ? 'ring-2 ring-primary-500' : ''}`}>
+          <div className="flex items-center gap-2">
+            <Key className="h-5 w-5 text-purple-600" />
+            <h2 className="text-xl font-bold text-gray-900">Google Gemini 配置</h2>
+            {config.ai_provider === 'gemini' && (
+              <span className="px-2 py-1 text-xs font-medium bg-primary-100 text-primary-700 rounded-full">当前使用</span>
+            )}
+          </div>
+
+          {/* PDF support highlight */}
+          <div className="bg-green-50 border border-green-200 rounded-lg p-3">
+            <p className="text-sm text-green-800">
+              ✅ Gemini 支持原生 PDF 理解，可直接处理 PDF 文件（最多 1000 页），完整保留图片、表格、公式等内容。
+            </p>
+          </div>
+
+          {/* API Key */}
+          <div>
+            <label className="block font-medium text-gray-900 mb-2">
+              API Key
+            </label>
+            <div className="relative">
+              <input
+                type={showApiKeys.gemini ? 'text' : 'password'}
+                value={config.gemini_api_key || ''}
+                onChange={(e) => handleChange('gemini_api_key', e.target.value)}
+                placeholder="AIza..."
+                className="w-full px-4 py-2 pr-10 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+              />
+              <button
+                type="button"
+                onClick={() => toggleApiKeyVisibility('gemini')}
+                className="absolute right-3 top-1/2 -translate-y-1/2 text-gray-400 hover:text-gray-600"
+              >
+                {showApiKeys.gemini ? <EyeOff className="h-5 w-5" /> : <Eye className="h-5 w-5" />}
+              </button>
+            </div>
+            <p className="text-sm text-gray-500 mt-1">从 https://aistudio.google.com/apikey 获取</p>
+          </div>
+
+          {/* Base URL (optional) */}
+          <div>
+            <label className="block font-medium text-gray-900 mb-2">
+              Base URL (可选)
+            </label>
+            <div className="relative">
+              <LinkIcon className="absolute left-3 top-1/2 -translate-y-1/2 h-5 w-5 text-gray-400" />
+              <input
+                type="text"
+                value={config.gemini_base_url}
+                onChange={(e) => handleChange('gemini_base_url', e.target.value)}
+                placeholder="https://generativelanguage.googleapis.com（留空使用默认）"
+                className="w-full pl-10 px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+              />
+            </div>
+            <p className="text-xs text-gray-500 mt-1">
+              可配置自定义代理或中转服务（支持 Key 轮训等）。留空则使用 Google 官方 API
+            </p>
+          </div>
+
+          {/* Model */}
+          <div>
+            <label className="block font-medium text-gray-900 mb-2">
+              模型
+            </label>
+            <input
+              type="text"
+              list="gemini-models"
+              value={config.gemini_model}
+              onChange={(e) => handleChange('gemini_model', e.target.value)}
+              placeholder="gemini-2.0-flash-exp"
+              className="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-primary-500 focus:border-transparent font-mono text-sm"
+            />
+            <datalist id="gemini-models">
+              <option value="gemini-2.0-flash-exp">gemini-2.0-flash-exp (最新，推荐)</option>
+              <option value="gemini-1.5-pro">gemini-1.5-pro (最强)</option>
+              <option value="gemini-1.5-flash">gemini-1.5-flash (快速)</option>
+              <option value="gemini-1.0-pro">gemini-1.0-pro</option>
+            </datalist>
+            <p className="text-xs text-gray-500 mt-1">可输入自定义模型名称，或从建议中选择</p>
           </div>
         </div>