fix: address second code review findings

- Add context cancellation check between pagination pages in GetArtistReleaseGroups
  for responsive graceful shutdown during large discography fetches.
- Eliminate double DB query on cache hit by having GetCachedReleases return
  []ExternalRelease directly instead of just a count, avoiding a redundant
  second query in SyncArtistDiscography.
- Update cache_test.go to match new GetCachedReleases return type.
- Format main.go (pre-existing whitespace issue).

internal/musicbrainz/api.go

@@ -60,6 +60,12 @@ func (c *MusicBrainzClient) GetArtistReleaseGroups(ctx context.Context, artistMB
 		if offset+len(parsed.ReleaseGroups) >= parsed.Count {
 			break
 		}
+
+		// Check context cancellation between pages for responsive shutdown.
+		if err := ctx.Err(); err != nil {
+			return nil, fmt.Errorf("fetch release groups for artist %s: %w", artistMBID, err)
+		}
+
 		offset += limit
 	}