add support for SQLite as an alternative database backend

Implement SQLite support using the pure Go `modernc.org/sqlite` driver and update the migration system to handle driver-specific schemas. Users can now choose between MySQL and SQLite by setting the `DB_DRIVER` environment variable.

internal/database/database.go

@@ -1,23 +1,41 @@
 package database
 
 import (
+	"fmt"
 	"time"
 
 	_ "github.com/go-sql-driver/mysql"
 	"github.com/jmoiron/sqlx"
+	_ "modernc.org/sqlite"
 
 	"emly-api-go/internal/config"
 )
 
 func Connect(cfg *config.Config) (*sqlx.DB, error) {
-	db, err := sqlx.Connect("mysql", cfg.DSN)
-	if err != nil {
-		return nil, err
-	}
+	var db *sqlx.DB
+	var err error
 
-	db.SetMaxOpenConns(cfg.MaxOpenConns)
-	db.SetMaxIdleConns(cfg.MaxIdleConns)
-	db.SetConnMaxLifetime(time.Duration(cfg.ConnMaxLifetime) * time.Minute)
+	switch cfg.Driver {
+	case "sqlite":
+		db, err = sqlx.Connect("sqlite", cfg.DSN)
+		if err != nil {
+			return nil, err
+		}
+		// Enable foreign key support (disabled by default in SQLite)
+		if _, err = db.Exec("PRAGMA foreign_keys = ON"); err != nil {
+			return nil, fmt.Errorf("sqlite: enable foreign_keys: %w", err)
+		}
+	case "mysql":
+		db, err = sqlx.Connect("mysql", cfg.DSN)
+		if err != nil {
+			return nil, err
+		}
+		db.SetMaxOpenConns(cfg.MaxOpenConns)
+		db.SetMaxIdleConns(cfg.MaxIdleConns)
+		db.SetConnMaxLifetime(time.Duration(cfg.ConnMaxLifetime) * time.Minute)
+	default:
+		return nil, fmt.Errorf("unsupported DB_DRIVER %q: must be mysql or sqlite", cfg.Driver)
+	}
 
 	return db, nil
 }