feat: expose LLM error reason in /api/ask response and UI

- Add llm_error field to AskResponse so users know why AI summarisation was skipped
- Show orange warning banner in frontend when LLM is not configured or call fails
- Update AskEndpoint tests to assert llm_error presence

backend/models/api.py

@@ -92,3 +92,4 @@ class AskResponse(BaseModel):
     events: list[AskEventRef]
     query_info: dict
     llm_used: bool
+    llm_error: str | None = None