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)

docs/plans/2026-05-21-musicbrainz-provider.md

@@ -84,12 +84,12 @@ Implement a MusicBrainz API provider with strict 1 request/second rate limiting,
 - [x] run tests - must pass before next task
 
 ### Task 5: Wire up provider in application entry point
-- [ ] update `cmd/naviwatcher/main.go` to initialize MusicBrainz client
-- [ ] add MusicBrainz client to application context/dependencies
-- [ ] ensure graceful shutdown includes closing HTTP client connections
-- [ ] update config validation to ensure MusicBrainz.UserAgent is set
-- [ ] write tests for main.go integration (startup/shutdown)
-- [ ] run tests - must pass before next task
+- [x] update `cmd/naviwatcher/main.go` to initialize MusicBrainz client
+- [x] add MusicBrainz client to application context/dependencies
+- [x] ensure graceful shutdown includes closing HTTP client connections
+- [x] update config validation to ensure MusicBrainz.UserAgent is set
+- [x] write tests for main.go integration (startup/shutdown)
+- [x] run tests - must pass before next task
 
 ### Task 6: Verify acceptance criteria and run full test suite
 - [ ] verify all requirements from Overview are implemented