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.go

@@ -3,14 +3,24 @@ package main
 import (
 	"context"
 	"flag"
+	"fmt"
 	"log"
 	"os"
 	"os/signal"
 	"syscall"
 
 	"naviwatcher/internal/config"
+	"naviwatcher/internal/database"
+	"naviwatcher/internal/musicbrainz"
 )
 
+// App holds all application dependencies for clean shutdown and testability.
+type App struct {
+	cfg            *config.Config
+	db             *database.DB
+	mbClient       *musicbrainz.MusicBrainzClient
+}
+
 const defaultConfigPath = "config.yaml"
 
 func main() {
@@ -35,20 +45,57 @@ func main() {
 	go func() {
 		sig := <-sigCh
 		log.Printf("Received signal %v, shutting down...", sig)
-		cancel()
-	}()
+		cancel()}()
 
-	if err := run(ctx, cfg); err != nil {
+	app, err := NewApp(ctx, cfg)
+	if err != nil {
+		log.Fatalf("Failed to initialize application: %v", err)
+	}
+	defer app.Close()
+
+	if err := app.run(ctx); err != nil {
 		log.Fatalf("Application error: %v", err)
 	}
 
 	log.Println("NaviWatcher stopped.")
 }
 
-func run(ctx context.Context, cfg *config.Config) error {
-	// Main application loop — blocks until context is cancelled.
-	// Business logic will be added in future tasks.
-	<-ctx.Done()
+// NewApp initializes all application components: config, database, and MusicBrainz client.
+func NewApp(ctx context.Context, cfg *config.Config) (*App, error) {
+	// Initialize database (uses default path or could be made configurable).
+	db, err := database.New("naviwatcher.db")
+	if err != nil {
+		return nil, fmt.Errorf("failed to initialize database: %w", err)
+	}
 
+	// Initialize MusicBrainz client with rate limiting.
+	mbClient := musicbrainz.NewClient(cfg.MusicBrainz)
+
+	log.Printf("MusicBrainz client initialized (user-agent: %s)", cfg.MusicBrainz.UserAgent)
+
+	return &App{
+		cfg:      cfg,
+		db:       db,
+		mbClient: mbClient,
+	}, nil
+}
+
+// Close cleans up all application resources in reverse order of initialization.
+func (a *App) Close() {
+	if a.mbClient != nil {
+		a.mbClient.Close()
+	}
+	if a.db != nil {
+		if err := a.db.Close(); err != nil {
+			log.Printf("Error closing database: %v", err)
+		}
+	}
+}
+
+func (a *App) run(ctx context.Context) error {
+	// Main application loop — blocks until context is cancelled.
+	// Business logic (scanner, notifier, web server) will be wired into
+	// separate goroutines here in future tasks.
+	<-ctx.Done()
 	return nil
 }