feat(09-01): implement GET /api/admin/overview endpoint — ADMIN-11

- New backend/api/admin/overview.py: aggregate stats (user_count,
  total_storage_bytes, doc_status) + last 10 audit entries in one payload
- Sub-router carries NO prefix (parent __init__.py carries /api/admin)
- Reuses _build_filtered_query_with_handles + _audit_to_dict_with_handles
  from api.audit — no new serializer or query logic
- Registered overview_router on admin parent router in __init__.py
- All 8 ADMIN-11 tests pass; existing admin and audit suites stay green

backend/api/admin/__init__.py

@@ -13,11 +13,13 @@ main.py continues to use:
   app.include_router(admin_router)
 """
 from fastapi import APIRouter
+from api.admin.overview import router as overview_router
 from api.admin.users import router as users_router
 from api.admin.quotas import router as quotas_router
 from api.admin.ai import router as ai_router
 
 router = APIRouter(prefix="/api/admin", tags=["admin"])
+router.include_router(overview_router)
 router.include_router(users_router)
 router.include_router(quotas_router)
 router.include_router(ai_router)