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/core/repository_exceptions.py

@@ -0,0 +1,26 @@
+"""
+Custom exceptions for the repository layer.
+
+These exceptions allow services and routes to handle database-level errors
+with proper semantics, without leaking SQLAlchemy internals.
+"""
+
+
+class RepositoryError(Exception):
+    """Base for all repository-layer errors."""
+
+
+class DuplicateEntryError(RepositoryError):
+    """Raised on unique constraint violations. Maps to HTTP 409 Conflict."""
+
+
+class IntegrityConstraintError(RepositoryError):
+    """Raised on FK or check constraint violations."""
+
+
+class RecordNotFoundError(RepositoryError):
+    """Raised when an expected record doesn't exist."""
+
+
+class InvalidInputError(RepositoryError):
+    """Raised on bad pagination params, invalid UUIDs, or other invalid inputs."""