fix: use separate except clauses for JWT verification fallback

ruff format strips parentheses from `except (A, B):`, turning it into
Python 2 comma syntax that only catches the first exception. Use
separate except clauses so PyJWKClientError is actually caught.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/app/core/auth.py

@@ -71,7 +71,9 @@ def _verify_jwt(token: str) -> dict | None:
                 algorithms=["RS256"],
                 audience="authenticated",
             )
-        except jwt.InvalidTokenError, PyJWKClientError:
+        except jwt.InvalidTokenError:
+            pass
+        except PyJWKClientError:
             pass
     return _verify_jwt_hs256(token)