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/sync.go

@@ -32,17 +32,17 @@ func SyncArtistDiscography(
 	}
 
 	// Step 1: Check cache.
-	cachedCount, err := GetCachedReleases(db, artistMBID, ttl)
+	cachedReleases, err := GetCachedReleases(db, artistMBID, ttl)
 	if err != nil {
 		return nil, fmt.Errorf("sync artist discography: cache check failed: %w", err)
 	}
 
 	// Step 2: If we have cached data, return it.
-	if cachedCount > 0 {
+	if len(cachedReleases) > 0 {
 		if err := ctx.Err(); err != nil {
 			return nil, fmt.Errorf("sync artist discography: %w", err)
 		}
-		return database.GetExternalReleasesByArtistWithCache(db, artistMBID, ttl)
+		return cachedReleases, nil
 	}
 
 	// Step 3: Cache miss — fetch from MusicBrainz API.