Refactor admin API into service and repository layers.

fix: Audit pass — critical bug fix + lazy import cleanup (qa/books/schemas_router)

refactor: Phase 6A+6B — top-level imports and Pydantic schema upgrades

feat: Phases 2B + 3B — semantic Q&A and vector store fixes

Refactor: 10/10 modularity — pipeline/ package, admin/routers/ split, guards single source of truth