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/application/streaming_service.py

@@ -72,16 +72,19 @@ class StreamingService:
         track = await self._tracks.get_by_id(track_id)
         if track is None:
             raise NotFoundError("Track not found.")
+        storage_uri = track.storage_uri
+        if storage_uri is None:
+            raise NotFoundError("Track is not yet downloaded.")
 
-        stat = await self._storage.stat(track.storage_uri)
+        stat = await self._storage.stat(storage_uri)
         total_size = stat.size
         content_type = stat.content_type or _FORMAT_CONTENT_TYPE.get(
-            track.file_format.lower(), "application/octet-stream"
+            (track.file_format or "").lower(), "application/octet-stream"
         )
 
         start, end, is_partial = _parse_range(range_header, total_size)
 
-        stream, _ = await self._storage.open_range(track.storage_uri, start, end)
+        stream, _ = await self._storage.open_range(storage_uri, start, end)
 
         actual_end = end if end is not None else total_size - 1
         content_length = actual_end - start + 1