feat: wire up MusicBrainz provider in application entry point
- Add App struct with Config, DB, and MusicBrainz client dependencies
- NewApp() initializes database and MusicBrainz client from config
- App.Close() cleans up resources (MB client + DB) on shutdown
- App.run() blocks until context cancelled (goroutine-ready for future tasks)
- Config validation already requires musicbrainz.user_agent
- Add 6 tests: graceful shutdown, config integration, NewApp creation,
shutdown cleanup, app.run shutdown, UserAgent validation
- All tests pass (6/6 in cmd, full suite green)
Vladimir Zagainov
da49d12bb6