Add mood scanner and surprise me features to discover page

Add mood_energy and mood_valence sliders that inject mood context into
AI recommendation prompts. Add "Surprise Me" button that generates
recommendations from a creative, unexpected angle without requiring
any user input. Includes backend endpoints, schema updates, and
full frontend UI integration.

backend/app/api/endpoints/recommendations.py

@@ -31,6 +31,25 @@ async def generate(
     recs, remaining = await generate_recommendations(
         db, user, playlist_id=data.playlist_id, query=data.query, bandcamp_mode=data.bandcamp_mode,
         mode=data.mode, adventurousness=data.adventurousness, exclude=data.exclude, count=data.count,
+        mood_energy=data.mood_energy, mood_valence=data.mood_valence,
+    )
+
+    if not recs and remaining == 0:
+        raise HTTPException(status_code=429, detail="Weekly recommendation limit reached. Upgrade to Premium for unlimited.")
+
+    return RecommendationResponse(
+        recommendations=[RecommendationItem.model_validate(r) for r in recs],
+        remaining_this_week=remaining,
+    )
+
+
+@router.post("/surprise", response_model=RecommendationResponse)
+async def surprise(
+    user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    recs, remaining = await generate_recommendations(
+        db, user, query=None, mode="surprise", count=5
     )
 
     if not recs and remaining == 0: