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/metadata_service.py

@@ -79,9 +79,13 @@ class MetadataEnrichmentService:
         if track.metadata_status == "manual":
             log.info("enrich_skip_manual", track_id=str(track_id))
             return EnrichmentResult(track_id=track_id, status="skipped")
+        storage_uri = track.storage_uri
+        if storage_uri is None:
+            log.info("enrich_skip_remote", track_id=str(track_id))
+            return EnrichmentResult(track_id=track_id, status="skipped")
 
-        tags = await self._read_local(track.storage_uri)
-        match = await self._identify(track.storage_uri)
+        tags = await self._read_local(storage_uri)
+        match = await self._identify(storage_uri)
 
         # Merge order is tag-first by default — embedded tags fix the common
         # well-tagged offline case. But a *high-confidence* AcoustID match is the
@@ -125,7 +129,7 @@ class MetadataEnrichmentService:
         if album is not None:
             await self._resolve_cover(
                 album,
-                storage_uri=track.storage_uri,
+                storage_uri=storage_uri,
                 release_group_mbid=match.release_group_mbid if match else None,
             )
 
@@ -175,6 +179,8 @@ class MetadataEnrichmentService:
             return []
         if not self._acoustid.is_available() or not self._fingerprinter.is_available():
             return []
+        if track.storage_uri is None:
+            return []
         try:
             async with self._storage.as_local_path(track.storage_uri) as path:
                 fingerprint = await self._fingerprinter.calculate(path)