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/app/repositories/__init__.py

@@ -0,0 +1,39 @@
+# app/repositories/__init__.py
+"""Repository layer — all database access goes through these classes."""
+
+from app.repositories.oauth_account import OAuthAccountRepository, oauth_account_repo
+from app.repositories.oauth_authorization_code import (
+    OAuthAuthorizationCodeRepository,
+    oauth_authorization_code_repo,
+)
+from app.repositories.oauth_client import OAuthClientRepository, oauth_client_repo
+from app.repositories.oauth_consent import OAuthConsentRepository, oauth_consent_repo
+from app.repositories.oauth_provider_token import (
+    OAuthProviderTokenRepository,
+    oauth_provider_token_repo,
+)
+from app.repositories.oauth_state import OAuthStateRepository, oauth_state_repo
+from app.repositories.organization import OrganizationRepository, organization_repo
+from app.repositories.session import SessionRepository, session_repo
+from app.repositories.user import UserRepository, user_repo
+
+__all__ = [
+    "UserRepository",
+    "user_repo",
+    "OrganizationRepository",
+    "organization_repo",
+    "SessionRepository",
+    "session_repo",
+    "OAuthAccountRepository",
+    "oauth_account_repo",
+    "OAuthAuthorizationCodeRepository",
+    "oauth_authorization_code_repo",
+    "OAuthClientRepository",
+    "oauth_client_repo",
+    "OAuthConsentRepository",
+    "oauth_consent_repo",
+    "OAuthProviderTokenRepository",
+    "oauth_provider_token_repo",
+    "OAuthStateRepository",
+    "oauth_state_repo",
+]