fix: add missing exception chaining in _verify_via_api

Self-audit caught: _verify_local had 'from e' on all 3 except
handlers, but _verify_via_api's catch-all was missing it. Original
Supabase error traceback was being lost on API fallback failures.

289 tests pass.

Author: DevanshuNEU

backend/services/auth.py

@@ -97,7 +97,7 @@ def _verify_via_api(self, token: str) -> Dict[str, Any]:
             raise
         except Exception as e:
             logger.debug("API-based JWT verification failed", error=str(e))
-            raise AuthenticationError("Token verification failed")
+            raise AuthenticationError("Token verification failed") from e
 
     async def signup(self, email: str, password: str, github_username: Optional[str] = None) -> Dict[str, Any]:
         """