Implements GET/DELETE /api/library, POST /api/library/{id}/reingest,
POST /api/library/scan, and GET /api/library/{id}/status. Adds FastAPI
app factory with lifespan migrations, BM25 singleton wiring, get_db
dependency, ingest task registry with cf-orch/BackgroundTasks fallback,
and placeholder search/chat routers. All 5 new tests pass (14 total).
