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_admin.py

@@ -147,7 +147,7 @@ class TestAdminCreateUser:
             headers={"Authorization": f"Bearer {superuser_token}"},
         )
 
-        assert response.status_code == status.HTTP_404_NOT_FOUND
+        assert response.status_code == status.HTTP_409_CONFLICT
 
 
 class TestAdminGetUser:
@@ -565,7 +565,7 @@ class TestAdminCreateOrganization:
             headers={"Authorization": f"Bearer {superuser_token}"},
         )
 
-        assert response.status_code == status.HTTP_404_NOT_FOUND
+        assert response.status_code == status.HTTP_409_CONFLICT
 
 
 class TestAdminGetOrganization: