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.

.env.example

@@ -6,13 +6,20 @@ API_DOMAIN=api.esempio.com
 ACME_EMAIL=tua@email.com
 MYSQL_ROOT_PASSWORD=password-sicura
 
-# DB Settings (usato per sviluppo locale; in Docker il DSN è costruito dal compose)
+# DB Settings
+# DB_DRIVER: "mysql" (default) o "sqlite"
+DB_DRIVER=mysql
+
+# MySQL
 DB_DSN=root:secret@tcp(127.0.0.1:3306)/emly?parseTime=true&loc=UTC
 DB_MAX_OPEN_CONNS=25
 DB_MAX_IDLE_CONNS=5
 DB_CONN_MAX_LIFETIME=5
 DATABASE_NAME=emly
 
+# SQLite (usare invece di MySQL: DB_DRIVER=sqlite, DB_DSN=./data.db, DATABASE_NAME non necessario)
+# DB_DSN=./data.db
+
 # API Keys
 API_KEY=key-one
 ADMIN_KEY=admin-key-one