fix: empty track_uris guard, None duration handling, README env block

spotify_client.py

@@ -46,6 +46,8 @@ def get_client() -> spotipy.Spotify:
 
 
 def _format_duration(ms: int) -> str:
+    if ms is None:
+        return "unknown"
     seconds = ms // 1000
     minutes, seconds = divmod(seconds, 60)
     return f"{minutes}:{seconds:02d}"
@@ -154,6 +156,8 @@ def create_playlist(name: str, description: str = "", public: bool = False) -> d
 
 
 def add_tracks_to_playlist(playlist_id: str, track_uris: list[str]) -> dict:
+    if not track_uris:
+        raise ValueError("track_uris must not be empty")
     sp = get_client()
     for i in range(0, len(track_uris), 100):
         sp.playlist_add_items(playlist_id, track_uris[i : i + 100])