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/api/test_permissions_security.py

@@ -11,7 +11,7 @@ These tests prevent unauthorized access and privilege escalation.
 import pytest
 from httpx import AsyncClient
 
-from app.crud.user import user as user_crud
+from app.repositories.user import user_repo as user_crud
 from app.models.organization import Organization
 from app.models.user import User