fix(audio): recover clip serverFileId from audio_file_url on project load

3 fixes for audio clips losing sound after save/reload:
1. deserializeTracksList: clip-level serverFileId falls back
   to extracting from audio_file_url when server_file_id missing
2. serializeTracksList: add server_file_id to track output
   so track-level fallback survives round-trip
3. upgrade_project_json_if_needed: include server_file_id
   in source_data during legacy project upgrade

app/api/v1/projects.py

@@ -38,6 +38,7 @@ def upgrade_project_json_if_needed(project_data: dict) -> dict:
                 "clip_start_offset_bars": 0.0,
                 "source_data": {
                     "audio_file_url": f"/static/audio/uploads/{t.get('serverFileId')}" if t.get("serverFileId") else "",
+                    "server_file_id": t.get("serverFileId") or None,
                     "sample_rate": 44100,
                     "channels": 2,
                     "gain": 1.0