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)

cmd/naviwatcher/main_test.go

@@ -13,9 +13,29 @@ func TestRun_GracefulShutdown(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	cfg := &config.Config{}
-	if err := run(ctx, cfg); err != nil {
-		t.Fatalf("run returned error: %v", err)
+	cfg := &config.Config{
+		Server: config.ServerConfig{
+			Host: "127.0.0.1",
+			Port: 9090,
+		},
+		Navidrome: config.NavidromeConfig{
+			URL:      "http://localhost:4533",
+			User:     "test",
+			Password: "test",
+		},
+		MusicBrainz: config.MusicBrainzConfig{
+			UserAgent: "NaviWatcher/1.0 ( test@example.com )",
+		},
+	}
+
+	app, err := NewApp(ctx, cfg)
+	if err != nil {
+		t.Fatalf("NewApp returned error: %v", err)
+	}
+	defer app.Close()
+
+	if err := app.run(ctx); err != nil {
+		t.Fatalf("app.run returned error: %v", err)
 	}
 }
 
@@ -54,3 +74,127 @@ musicbrainz:
 		t.Errorf("expected navidrome url http://localhost:4533, got %q", cfg.Navidrome.URL)
 	}
 }
+
+func TestNewApp_CreatesMusicBrainzClient(t *testing.T) {
+	// Verify that NewApp initializes the MusicBrainz client from config.
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	_ = dbPath // database.New uses a hardcoded path in this version; we test the client creation.
+
+	cfg := &config.Config{
+		Server: config.ServerConfig{
+			Host: "127.0.0.1",
+			Port: 9090,
+		},
+		Navidrome: config.NavidromeConfig{
+			URL:      "http://localhost:4533",
+			User:     "test",
+			Password: "test",
+		},
+		MusicBrainz: config.MusicBrainzConfig{
+			UserAgent: "NaviWatcher/1.0 ( test@example.com )",
+		},
+	}
+
+	ctx := context.Background()
+	app, err := NewApp(ctx, cfg)
+	if err != nil {
+		t.Fatalf("NewApp returned error: %v", err)
+	}
+	defer app.Close()
+
+	if app.mbClient == nil {
+		t.Fatal("expected MusicBrainz client to be initialized, got nil")
+	}
+	if app.db == nil {
+		t.Fatal("expected database to be initialized, got nil")
+	}
+	if app.cfg != cfg {
+		t.Fatal("expected app.cfg to be the config passed to NewApp")
+	}
+}
+
+func TestNewApp_GracefulShutdown(t *testing.T) {
+	// Verify that App.Close() cleans up resources without error.
+	cfg := &config.Config{
+		Server: config.ServerConfig{
+			Host: "127.0.0.1",
+			Port: 9090,
+		},
+		Navidrome: config.NavidromeConfig{
+			URL:      "http://localhost:4533",
+			User:     "test",
+			Password: "test",
+		},
+		MusicBrainz: config.MusicBrainzConfig{
+			UserAgent: "NaviWatcher/1.0 ( test@example.com )",
+		},
+	}
+
+	ctx := context.Background()
+	app, err := NewApp(ctx, cfg)
+	if err != nil {
+		t.Fatalf("NewApp returned error: %v", err)
+	}
+
+	// Close should not panic or return error.
+	app.Close()
+}
+
+func TestAppRun_GracefulShutdown(t *testing.T) {
+	// Verify that app.run() returns nil when context is cancelled.
+	cfg := &config.Config{
+		Server: config.ServerConfig{
+			Host: "127.0.0.1",
+			Port: 9090,
+		},
+		Navidrome: config.NavidromeConfig{
+			URL:      "http://localhost:4533",
+			User:     "test",
+			Password: "test",
+		},
+		MusicBrainz: config.MusicBrainzConfig{
+			UserAgent: "NaviWatcher/1.0 ( test@example.com )",
+		},
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	app, err := NewApp(ctx, cfg)
+	if err != nil {
+		t.Fatalf("NewApp returned error: %v", err)
+	}
+	defer app.Close()
+
+	// Cancel the context to trigger shutdown.
+	cancel()
+
+	if err := app.run(ctx); err != nil {
+		t.Fatalf("app.run returned error: %v", err)
+	}
+}
+
+func TestMusicBrainzUserAgentValidation(t *testing.T) {
+	// Verify that config validation requires MusicBrainz.UserAgent to be set.
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+
+	yaml := `server:
+  host: "127.0.0.1"
+  port: 9090
+navidrome:
+  url: "http://localhost:4533"
+  user: "test"
+  password: "test"
+musicbrainz:
+  user_agent: ""
+`
+	if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+		t.Fatalf("failed to write config: %v", err)
+	}
+
+	_, err := config.LoadConfig(path)
+	if err == nil {
+		t.Fatal("expected config validation error for empty musicbrainz.user_agent, got nil")
+	}
+}