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/domain/entities/track.py

@@ -9,6 +9,8 @@ from dataclasses import dataclass
 class Artist:
     id: uuid.UUID
     name: str
+    source: str | None
+    source_id: str | None
     created_at: dt.datetime
     updated_at: dt.datetime
 
@@ -19,9 +21,9 @@ class Track:
     title: str
     artist_id: uuid.UUID
     album_id: uuid.UUID | None
-    storage_uri: str
-    file_format: str
-    file_size: int
+    storage_uri: str | None
+    file_format: str | None
+    file_size: int | None
     source: str
     source_id: str
     duration_seconds: int | None
@@ -31,5 +33,6 @@ class Track:
     metadata_status: str
     metadata_error: str | None
     enriched_at: dt.datetime | None
+    availability: str
     created_at: dt.datetime
     updated_at: dt.datetime