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>

internal/server/server.go

@@ -6,6 +6,7 @@ import (
 	"strconv"
 	"time"
 
+	"music-server/internal/backend"
 	"music-server/internal/db"
 	"music-server/internal/logging"
 	"net/http"
@@ -15,7 +16,9 @@ import (
 
 type Server struct {
 	port         int
+	db          *db.Database
 	tokenHandler *TokenHandler
+	httpServer  *http.Server
 }
 
 var (
@@ -30,7 +33,9 @@ var (
 	logJSON        = os.Getenv("LOG_JSON") == "true"
 )
 
-func NewServer() *http.Server {
+// NewServerInstance creates a new Server instance with all dependencies initialized.
+// Use this for dependency injection and proper lifecycle management.
+func NewServerInstance() *Server {
 	// Initialize logger
 	if logLevel == "" {
 		logLevel = "info"
@@ -40,15 +45,45 @@ func NewServer() *http.Server {
 	logger := logging.GetLogger()
 
 	port, _ := strconv.Atoi(os.Getenv("PORT"))
-	
-	// Initialize token handler
-	tokenHandler := NewTokenHandler()
-	
-	NewServer := &Server{
+
+	// Validate required environment variables
+	if host == "" || dbPort == "" || username == "" || password == "" || dbName == "" || musicPath == "" || charactersPath == "" {
+		logging.GetLogger().Fatal("Invalid settings - missing required environment variables")
+	}
+
+	// Create database instance
+	database, err := db.NewDatabase(host, dbPort, username, password, dbName)
+	if err != nil {
+		logging.GetLogger().Fatal("Failed to initialize database", zap.String("error", err.Error()))
+	}
+
+	// Run migrations using the new method
+	if err := database.RunMigrations(); err != nil {
+		logging.GetLogger().Fatal("Migration failed", zap.String("error", err.Error()))
+	}
+
+	// Initialize backend package with database pool
+	backend.InitBackend(database.Pool)
+
+	// Initialize token handler with database pool
+	tokenHandler := NewTokenHandler(database.Pool)
+
+	// Create the server instance
+	appServer := &Server{
 		port:         port,
+		db:          database,
 		tokenHandler: tokenHandler,
 	}
 
+	// Create the HTTP server
+	appServer.httpServer = &http.Server{
+		Addr:         fmt.Sprintf(":%d", port),
+		Handler:      appServer.RegisterRoutes(),
+		IdleTimeout:  time.Minute,
+		ReadTimeout:  10 * time.Second,
+		WriteTimeout: 30 * time.Second,
+	}
+
 	logger.Info("Starting server",
 		zap.String("host", host),
 		zap.String("dbPort", dbPort),
@@ -61,23 +96,21 @@ func NewServer() *http.Server {
 		zap.String("charactersPath", charactersPath),
 	)
 
-	//conf.SetupDb()
-	if host == "" || dbPort == "" || username == "" || password == "" || dbName == "" || musicPath == "" || charactersPath == "" {
-		logging.GetLogger().Fatal("Invalid settings - missing required environment variables")
-	}
-
-	db.Migrate_db(host, dbPort, username, password, dbName)
-
-	db.InitDB(host, dbPort, username, password, dbName)
-
-	// Declare Server config
-	server := &http.Server{
-		Addr:         fmt.Sprintf(":%d", NewServer.port),
-		Handler:      NewServer.RegisterRoutes(),
-		IdleTimeout:  time.Minute,
-		ReadTimeout:  10 * time.Second,
-		WriteTimeout: 30 * time.Second,
-	}
-
-	return server
+	return appServer
+}
+
+// HTTPServer returns the underlying http.Server for serving HTTP requests.
+func (s *Server) HTTPServer() *http.Server {
+	return s.httpServer
+}
+
+// DB returns the database instance for dependency injection.
+func (s *Server) DB() *db.Database {
+	return s.db
+}
+
+// NewServer creates a new HTTP server (deprecated, use NewServerInstance instead).
+// This function is kept for backward compatibility.
+func NewServer() *http.Server {
+	return NewServerInstance().HTTPServer()
 }