feat: Add MySQL database support (#3024)

* feat: Add MySQL database support

- Add MySQL database support with environment-based configuration
- Fix MySQL compatibility issue with 'key' column name
- Maintain SQLite as default database
- Add proper validation for MySQL configuration
- Test and verify compatibility with existing database
- Replaced raw SQL queries using JSON_EACH functions with standard GORM queries
- Modified functions to handle JSON parsing in Go code instead of database since JSON_EACH is not available on MySQL or MariaDB:
  - getAllEmails()
  - GetClientTrafficByID()
  - getFallbackMaster()
  - MigrationRemoveOrphanedTraffics()

The system now supports both MySQL and SQLite databases, with SQLite remaining as the default option. MySQL connection is only used when explicitly configured through environment variables.

* refactor: prefix env variables of database with XUI_ to support direct environment usage without .env file

All database configuration environment variables now start with the XUI_ prefix to avoid conflicts and allow configuration via system-level environment variables, not just the .env file.

.env.example

@@ -0,0 +1,8 @@
+XUI_DB_CONNECTION=sqlite
+
+# If DB connection is "mysql"
+# XUI_DB_HOST=127.0.0.1
+# XUI_DB_PORT=3306
+# XUI_DB_DATABASE=xui
+# XUI_DB_USERNAME=root
+# XUI_DB_PASSWORD=