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/db/database.go

@@ -0,0 +1,121 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"music-server/internal/logging"
+
+	"github.com/golang-migrate/migrate/v4"
+	"github.com/golang-migrate/migrate/v4/database/postgres"
+	_ "github.com/golang-migrate/migrate/v4/database/postgres"
+	"github.com/golang-migrate/migrate/v4/source/iofs"
+	"github.com/jackc/pgx/v5/pgxpool"
+	_ "github.com/lib/pq"
+	"go.uber.org/zap"
+)
+
+// Database holds the database connection pool and context
+type Database struct {
+	Pool *pgxpool.Pool
+	Ctx  context.Context
+}
+
+// NewDatabase creates a new Database instance with connection pool
+func NewDatabase(host, port, user, password, dbname string) (*Database, error) {
+	ctx := context.Background()
+	psqlInfo := fmt.Sprintf("host=%s port=%s user=%s password=%s dbname=%s sslmode=disable",
+		host, port, user, password, dbname)
+
+	logging.GetLogger().Debug("Database connection info",
+		zap.String("host", host),
+		zap.String("port", port),
+		zap.String("dbname", dbname))
+
+	pool, err := pgxpool.New(ctx, psqlInfo)
+	if err != nil {
+		return nil, fmt.Errorf("unable to connect to database: %w", err)
+	}
+
+	// Test connection
+	var success string
+	err = pool.QueryRow(ctx, "select 'Successfully connected!'").Scan(&success)
+	if err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("database query failed: %w", err)
+	}
+
+	logging.GetLogger().Info("Database connected", zap.String("status", success))
+
+	return &Database{Pool: pool, Ctx: ctx}, nil
+}
+
+// Close closes the database connection pool
+func (db *Database) Close() {
+	if db.Pool != nil {
+		logging.GetLogger().Info("Closing database connection")
+		db.Pool.Close()
+	}
+}
+
+// RunMigrations runs all pending database migrations to the latest version.
+// Uses the existing pool to extract connection details.
+func (db *Database) RunMigrations() error {
+	// Extract connection info from pool config
+	connConfig := db.Pool.Config().ConnConfig
+	migrationURL := fmt.Sprintf("postgres://%s:%s@%s:%d/%s?sslmode=disable",
+		connConfig.User,
+		connConfig.Password,
+		connConfig.Host,
+		connConfig.Port,
+		connConfig.Database)
+
+	logging.GetLogger().Debug("Migration info", zap.String("url", migrationURL))
+
+	sqlDb, err := sql.Open("postgres", migrationURL)
+	if err != nil {
+		return fmt.Errorf("failed to open database for migration: %w", err)
+	}
+	defer sqlDb.Close()
+
+	driver, err := postgres.WithInstance(sqlDb, &postgres.Config{})
+	if err != nil {
+		return fmt.Errorf("failed to create migration driver: %w", err)
+	}
+
+	files, err := iofs.New(MigrationsFs, "migrations")
+	if err != nil {
+		return fmt.Errorf("failed to create migration files: %w", err)
+	}
+
+	m, err := migrate.NewWithInstance("iofs", files, "postgres", driver)
+	if err != nil {
+		return fmt.Errorf("failed to create migrator: %w", err)
+	}
+
+	// Get current version for logging
+	version, _, err := m.Version()
+	if err != nil {
+		logging.GetLogger().Error("Failed to get migration version", zap.String("error", err.Error()))
+	}
+
+	logging.GetLogger().Info("Migration version before", zap.Uint("version", version))
+
+	// Run all pending migrations to latest version
+	err = m.Up()
+	if err != nil {
+		if err == migrate.ErrNoChange {
+			logging.GetLogger().Info("Database already up to date")
+		} else {
+			return fmt.Errorf("migration failed: %w", err)
+		}
+	} else {
+		// Get new version after migration
+		versionAfter, _, _ := m.Version()
+		logging.GetLogger().Info("Migrated to version", zap.Uint("version", versionAfter))
+	}
+
+	logging.GetLogger().Info("Migration completed")
+	return nil
+}