fix: harden request handling, pipeline containment, and startup boundaries

backend/app/api/recommendations.py

@@ -8,7 +8,7 @@ import structlog
 from fastapi import APIRouter, HTTPException, Request
 from fastapi.responses import StreamingResponse
 
-from app.adapters.spotify.errors import SpotifyError
+from app.adapters.spotify.errors import SpotifyAuthenticationError, SpotifyError
 from app.adapters.spotify.session import SpotifySession
 from app.api.routes import resolve_session
 from app.api.schemas import (
@@ -69,7 +69,6 @@ async def recommendations(
 
     return StreamingResponse(
         _stream_lines(
-            request,
             pipeline,
             resolved.session_id,
             request_id,
@@ -101,6 +100,9 @@ async def create_playlist(
             "Music discovery selected by the listener.",
         )
         await writer.add_tracks_to_playlist(playlist.id, payload.track_uris)
+    except SpotifyAuthenticationError as error:
+        structlog.get_logger().warning("playlist_write_failed", error_type=type(error).__name__)
+        raise HTTPException(status_code=401, detail="Spotify authentication expired") from error
     except (SpotifyError, ValueError) as error:
         structlog.get_logger().warning("playlist_write_failed", error_type=type(error).__name__)
         raise HTTPException(status_code=502, detail="Spotify playlist creation failed") from error
@@ -108,7 +110,6 @@ async def create_playlist(
 
 
 async def _stream_lines(
-    request: Request,
     pipeline: RecommendationPipeline,
     session_id: str,
     request_id: str,
@@ -127,8 +128,6 @@ async def _stream_lines(
     )
     try:
         async for event in event_stream:
-            if await request.is_disconnected():
-                return
             yield f"{_to_wire_event(event).model_dump_json()}\n"
     except asyncio.CancelledError:
         raise
@@ -137,12 +136,11 @@ async def _stream_lines(
             "recommendation_stream_failed",
             error_type=type(error).__name__,
         )
-        if not await request.is_disconnected():
-            failure = ErrorEvent(
-                code="recommendation_failed",
-                message="Recommendation could not be completed.",
-            )
-            yield f"{failure.model_dump_json()}\n"
+        failure = ErrorEvent(
+            code="recommendation_failed",
+            message="Recommendation could not be completed.",
+        )
+        yield f"{failure.model_dump_json()}\n"
     finally:
         await event_stream.aclose()