fix: surface Spotify quota exhaustion

Co-Authored-By: Claude <noreply@anthropic.com>

backend/tests/test_orchestrator.py

@@ -146,6 +146,30 @@ def test_intent_stage_failures_yield_one_typed_error_event() -> None:
     asyncio.run(run())
 
 
+def test_grounding_quota_yields_typed_error_instead_of_empty_results() -> None:
+    class QuotaCatalog(FakeCatalog):
+        """Exhaust the Spotify quota on the first grounding search."""
+
+        async def search_tracks(self, query: str, limit: int = 10) -> list[Track]:
+            self.search_call_count += 1
+            raise SpotifyQuotaExhaustedError(0.0, "QUOTA_EXCEEDED")
+
+    async def run() -> None:
+        track = _track("candidate", "Candidate Song")
+        catalog = QuotaCatalog(())
+        recommender = FakeRecommender([_intent(track)])
+
+        events = await _run_pipeline(catalog, recommender)
+
+        assert [event.type for event in events] == ["metadata", "error"]
+        assert isinstance(events[-1], PipelineErrorEvent)
+        assert events[-1].code == "quota_exhausted"
+        assert catalog.search_call_count == 1
+        assert recommender.rerank_call_count == 0
+
+    asyncio.run(run())
+
+
 def test_taste_failure_cancels_sibling_fetches() -> None:
     class FailingTasteCatalog(FakeCatalog):
         """Fail one taste request after all sibling requests have started."""