feat(library): lazy materialization foundation for remote tracks (§Phase1)

Adds nullable storage fields + availability column on tracks, remote
source/source_id identity on albums/artists, TrackRepository.materialize()
and get_or_create_remote() repos — groundwork for on-demand YTM library
(placeholders saved without audio, materialized in-place on first play).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/rest/media.py

@@ -65,7 +65,7 @@ async def download(
     if track is None:
         raise NotFoundError("Song not found.")
     result = await service.open_stream(track_id, None)
-    filename = f"{track.title}.{track.file_format}"
+    filename = f"{track.title}.{track.file_format or 'bin'}"
     headers = {
         "Content-Length": str(result.content_length),
         "Content-Disposition": f'attachment; filename="{filename}"',

app/api/rest/serializers.py

@@ -80,8 +80,8 @@ def song_dict(
         "albumId": encode_album(track.album_id) if track.album_id is not None else None,
         "artistId": encode_artist(track.artist_id),
         "coverArt": cover,
-        "size": track.file_size,
-        "contentType": content_type_for(track.file_format),
+        "size": track.file_size or 0,
+        "contentType": content_type_for(track.file_format or ""),
         "suffix": track.file_format,
         "duration": track.duration_seconds,
         "year": track.year,

app/api/schemas/track.py

@@ -14,14 +14,15 @@ class TrackOut(BaseModel):
     album_id: uuid.UUID | None
     album_title: str | None
     duration_seconds: int | None
-    file_format: str
-    file_size: int
+    file_format: str | None
+    file_size: int | None
     genre: str | None
     year: int | None
     track_number: int | None
     metadata_status: str
     metadata_error: str | None
     enriched_at: dt.datetime | None
+    availability: str
     source: str
     has_cover: bool
     created_at: dt.datetime

app/api/v1/tracks.py

@@ -54,6 +54,7 @@ async def _build_track_out(
             metadata_status=t.metadata_status,
             metadata_error=t.metadata_error,
             enriched_at=t.enriched_at,
+            availability=t.availability,
             source=t.source,
             has_cover=bool(t.album_id and albums.get(t.album_id) and albums[t.album_id].cover_path),
             created_at=t.created_at,
@@ -155,7 +156,8 @@ async def delete_track(
     if track is None:
         raise NotFoundError(f"Track {track_id} not found.")
     await track_repo.delete(track_id)
-    await storage.delete(track.storage_uri)
+    if track.storage_uri is not None:
+        await storage.delete(track.storage_uri)
     return Response(status_code=204)