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/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