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/schemas/recommendation.py

@@ -11,6 +11,8 @@ class RecommendationRequest(BaseModel):
     adventurousness: int = 3  # 1-5
     exclude: str | None = None  # comma-separated genres to exclude
     count: int = 5  # Number of recommendations (5, 10, 15, 20)
+    mood_energy: int | None = None  # 1-5, 1=chill, 5=energetic
+    mood_valence: int | None = None  # 1-5, 1=sad/dark, 5=happy/bright
 
 
 class RecommendationItem(BaseModel):