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/test_helpers.go

@@ -8,6 +8,9 @@ import (
 	"testing"
 	"time"
 
+	"music-server/internal/backend"
+	"music-server/internal/db"
+
 	"github.com/labstack/echo/v5"
 )
 
@@ -45,8 +48,23 @@ func StartTestServer(t *testing.T) *echo.Echo {
 		os.Setenv("LOG_JSON", "false")
 	}
 
+	// Initialize database for tests
+	db.TestSetupDB(t)
+	
+	// Initialize backend with the global Dbpool
+	// This ensures BackendRepo() and BackendCtx() are available
+	if db.Dbpool != nil {
+		backend.InitBackend(db.Dbpool)
+	}
+
 	// Create a Server instance and get its routes
-	s := &Server{}
+	s := &Server{
+		db: &db.Database{
+			Pool: db.Dbpool,
+			Ctx:  db.Ctx,
+		},
+		tokenHandler: NewTokenHandler(db.Dbpool),
+	}
 	handler := s.RegisterRoutes()
 
 	// Wrap the http.Handler in an echo.Echo