Add Last.fm import support

Users can import their top tracks from Last.fm by entering their username.
No OAuth required - uses the public Last.fm API with user.getTopTracks and
user.getInfo endpoints. Includes a preview feature to verify the username
and see sample tracks before importing. Supports configurable time periods
(all time, 7 days, 1/3/6/12 months). Free tier playlist limit enforced.

backend/app/core/config.py

@@ -28,6 +28,9 @@ class Settings(BaseSettings):
     STRIPE_PRICE_ID: str = ""
     STRIPE_WEBHOOK_SECRET: str = ""
 
+    # Last.fm
+    LASTFM_API_KEY: str = ""
+
     # Frontend
     FRONTEND_URL: str = "http://localhost:5173"