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.

go.mod

@@ -22,6 +22,7 @@ require (
 	golang.org/x/crypto v0.38.0
 	golang.org/x/text v0.25.0
 	google.golang.org/grpc v1.72.1
+	gorm.io/driver/mysql v1.5.7
 	gorm.io/driver/sqlite v1.5.7
 	gorm.io/gorm v1.25.12
 )
@@ -41,6 +42,7 @@ require (
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.26.0 // indirect
+	github.com/go-sql-driver/mysql v1.7.0 // indirect
 	github.com/go-task/slim-sprig/v3 v3.0.0 // indirect
 	github.com/google/btree v1.1.3 // indirect
 	github.com/google/pprof v0.0.0-20250501235452-c0086092b71a // indirect