feat: add key-free demo mode replaying recorded fixtures

backend/app/api/recommendations.py

@@ -1,4 +1,4 @@
-"""Authenticated recommendation streaming and playlist creation routes."""
+"""Recommendation streaming and playlist creation routes."""
 
 import asyncio
 from collections.abc import AsyncIterator, Callable
@@ -38,6 +38,10 @@ from app.pipeline.orchestrator import RecommendationPipeline
 from app.ports.protocols import MusicCatalog, PlaylistWriter
 
 SpotifyClientFactory = Callable[[SpotifySession], MusicCatalog | PlaylistWriter]
+MusicCatalogFactory = Callable[
+    [SpotifySession, str, tuple[PreviousRecommendation, ...]],
+    MusicCatalog,
+]
 
 router = APIRouter()
 
@@ -47,14 +51,12 @@ async def recommendations(
     request: Request,
     payload: RecommendationRequest,
 ) -> StreamingResponse:
-    """Stream one authenticated discovery response as NDJSON."""
+    """Stream one session-resolved discovery response as NDJSON."""
     resolved = resolve_session(request)
     if resolved is None:
         raise HTTPException(status_code=401, detail="Not authenticated")
 
     pipeline = cast(RecommendationPipeline, request.app.state.recommendation_pipeline)
-    factory = cast(SpotifyClientFactory, request.app.state.spotify_client_factory)
-    catalog = cast(MusicCatalog, factory(resolved.session))
     request_id = cast(str, request.state.request_id)
     history = tuple(ConversationTurn(turn.role, turn.content) for turn in payload.history)
     previous = tuple(
@@ -66,6 +68,8 @@ async def recommendations(
         )
         for item in payload.prior_recommendations
     )
+    factory = cast(MusicCatalogFactory, request.app.state.music_catalog_factory)
+    catalog = factory(resolved.session, payload.query, previous)
 
     return StreamingResponse(
         _stream_lines(
@@ -86,7 +90,7 @@ async def create_playlist(
     request: Request,
     payload: PlaylistCreateRequest,
 ) -> PlaylistCreateResponse:
-    """Create and fill one authenticated Spotify playlist."""
+    """Create and fill one live or simulated playlist."""
     resolved = resolve_session(request)
     if resolved is None:
         raise HTTPException(status_code=401, detail="Not authenticated")