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).

cmd/naviwatcher/main.go

@@ -16,9 +16,9 @@ import (
 
 // App holds all application dependencies for clean shutdown and testability.
 type App struct {
-	cfg            *config.Config
-	db             *database.DB
-	mbClient       *musicbrainz.MusicBrainzClient
+	cfg      *config.Config
+	db       *database.DB
+	mbClient *musicbrainz.MusicBrainzClient
 }
 
 const defaultConfigPath = "config.yaml"