feat: Remove global db.Dbpool with dependency injection (Phase 0)

- Add Database struct in internal/db/database.go with Pool, Ctx, and RunMigrations()
- Update server.go to use Database struct with NewServerInstance()
- Add backend.go with InitBackend(), BackendRepo(), BackendCtx(), BackendPool()
- Update music.go and sync.go to use BackendRepo() and BackendCtx() instead of db.Dbpool/db.Ctx
- Update token_handler.go to accept pool parameter
- Update routes.go to use s.db.Pool for middleware
- Update cmd/main.go to use NewServerInstance() and HTTPServer()
- Update test_helpers.go to initialize backend with test database
- Update test files to use backend.BackendPool() and backend.BackendCtx()

Benefits:
- Easier to mock database for unit tests
- Follows Go best practices (dependency injection)
- Better architecture with explicit dependencies
- RunMigrations() replaces old Migrate_db() function

Note: Global db.Dbpool and db.Ctx still exist in dbHelper.go for backward compatibility
with test_helpers.go, but production code no longer uses them.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

cmd/main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"context"
-	"music-server/internal/db"
 	"music-server/internal/logging"
 	"music-server/internal/server"
 	"net/http"
@@ -19,9 +18,11 @@ import (
 //	@description	This is a sample server Petstore server.
 //	@termsOfService	http://swagger.io/terms/
 
+//
 //	@contact.name	Sebastian Olsson
 //	@contact.email	zarnor91@gmail.com
 
+//
 //	@license.name	Apache 2.0
 //	@license.url	http://www.apache.org/licenses/LICENSE-2.0.html
 
@@ -34,16 +35,17 @@ func main() {
 	pprof.StartCPUProfile(f)
 	defer pprof.StopCPUProfile()*/
 
-	server := server.NewServer()
+	appServer := server.NewServerInstance()
+	httpServer := appServer.HTTPServer()
 
 	// Create a done channel to signal when the shutdown is complete
 	done := make(chan bool, 1)
 
 	// Run graceful shutdown in a separate goroutine
-	go gracefulShutdown(server, done)
+	go gracefulShutdown(appServer, httpServer, done)
 
-	logging.GetLogger().Info("Server starting", zap.String("address", server.Addr))
-	err := server.ListenAndServe()
+	logging.GetLogger().Info("Server starting", zap.String("address", httpServer.Addr))
+	err := httpServer.ListenAndServe()
 	if err != nil && err != http.ErrServerClosed {
 		logging.GetLogger().Fatal("HTTP server error", zap.String("error", err.Error()))
 	}
@@ -53,7 +55,7 @@ func main() {
 	logging.GetLogger().Info("Graceful shutdown complete")
 }
 
-func gracefulShutdown(apiServer *http.Server, done chan bool) {
+func gracefulShutdown(appServer *server.Server, httpServer *http.Server, done chan bool) {
 	// Create context that listens for the interrupt signal from the OS.
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
@@ -62,13 +64,17 @@ func gracefulShutdown(apiServer *http.Server, done chan bool) {
 	<-ctx.Done()
 
 	logging.GetLogger().Info("Shutting down gracefully, press Ctrl+C again to force")
-	db.CloseDb()
+	
+	// Close database connection
+	if appServer != nil && appServer.DB() != nil {
+		appServer.DB().Close()
+	}
 
 	// The context is used to inform the server it has 5 seconds to finish
 	// the request it is currently handling
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	if err := apiServer.Shutdown(ctx); err != nil {
+	if err := httpServer.Shutdown(ctx); err != nil {
 		logging.GetLogger().Error("Server forced to shutdown with error", zap.String("error", err.Error()))
 	}