完善文档与前端迁移，补充开源协议

docs/audit/backend-findings.md

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+# Backend Findings
+
+## Critical Findings
+
+### Schema lifecycle is unsafe
+
+- App startup still calls `create_all()`
+- Alembic metadata exists but the migration chain is effectively empty
+- This prevents controlled upgrades and rollbacks
+
+Files:
+
+- `backend/main.py`
+- `backend/database.py`
+- `backend/alembic/versions/.gitkeep`
+
+### Parsing tasks are not durable
+
+- Document ingestion runs inside FastAPI `BackgroundTasks`
+- Progress state lives in-process only
+- Process restarts or horizontal scaling can strand exams in `pending` or `processing`
+
+Files:
+
+- `backend/routers/exam.py`
+- `backend/services/progress_service.py`
+
+### Transaction boundaries are inconsistent
+
+- `get_db()` performs commit/rollback automatically
+- Routers and background tasks also call `commit()` directly
+- SSE endpoints keep a database dependency open for long-lived streams
+
+Files:
+
+- `backend/database.py`
+- `backend/routers/exam.py`
+
+## High-Priority Bugs
+
+### Admin config can destroy secrets
+
+- `GET /api/admin/config` masks API keys
+- `PUT /api/admin/config` persists whatever the frontend sends back
+- A round-trip save can replace the real secret with the masked placeholder
+
+Files:
+
+- `backend/routers/admin.py`
+
+### LLM service has import-time side effects
+
+- `LLMService()` is instantiated at module import time
+- Missing environment variables can break startup before DB-backed config is loaded
+
+Files:
+
+- `backend/services/llm_service.py`
+
+### Ingestion deduplication is race-prone
+
+- No unique DB constraint on `(exam_id, content_hash)`
+- Multiple append operations can race and insert duplicates
+
+Files:
+
+- `backend/models.py`
+- `backend/routers/exam.py`
+
+### Answer checking degrades incorrectly on infra failure
+
+- Short-answer grading failures are converted into zero scores
+- User mistake data can be polluted by provider outages or config errors
+
+Files:
+
+- `backend/services/llm_service.py`
+- `backend/routers/question.py`
+
+## Refactor Order
+
+1. Replace runtime schema creation with Alembic-first migrations.
+2. Move ingestion, config, and answer checking into service classes.
+3. Introduce explicit transaction boundaries and idempotent ingestion rules.
+4. Add durable task execution and real status/error semantics.
+5. Add integration tests for config round-trips, ingestion races, and answer normalization.