refactor(backend): enforce route→service→repo layered architecture

- introduce custom repository exception hierarchy (DuplicateEntryError,
  IntegrityConstraintError, InvalidInputError) replacing raw ValueError
- eliminate all direct repository imports and raw SQL from route layer
- add UserService, SessionService, OrganizationService to service layer
- add get_stats/get_org_distribution service methods replacing admin inline SQL
- fix timing side-channel in authenticate_user via dummy bcrypt check
- replace SHA-256 client secret fallback with explicit InvalidClientError
- replace assert with InvalidGrantError in authorization code exchange
- replace N+1 token revocation loops with bulk UPDATE statements
- rename oauth account token fields (drop misleading 'encrypted' suffix)
- add Alembic migration 0003 for token field column rename
- add 45 new service/repository tests; 975 passing, 94% coverage

backend/tests/e2e/test_admin_workflows.py

@@ -46,7 +46,7 @@ async def login_user(client, email: str, password: str = "SecurePassword123!"):
 
 async def create_superuser(e2e_db_session, email: str, password: str):
     """Create a superuser directly in the database."""
-    from app.crud.user import user as user_crud
+    from app.repositories.user import user_repo as user_crud
     from app.schemas.users import UserCreate
 
     user_in = UserCreate(