✨ feat(database): implement multi-database support with xdb

[seakee]

Summary

• Replace mysql package with xdb to support multiple databases: MySQL, PostgreSQL, SQLite, SQL Server, ClickHouse
• Adjust database configuration structure with new fields: port, charset, ssl_mode, timezone
• Update related config files and documentation

Changes

• 📦 Dependencies: Remove sk-pkg/mysql, add sk-pkg/xdb
• 🔧 Config: Add multi-database config fields in app/config/db.go
• ✨ Core: Implement unified multi-database initialization in bootstrap/db.go
• 🔧 Config Files: Update dev/local/prod config format
• 📝 Docs: Sync README and deployment/development guides

Testing

• MySQL connection test
• Config loading verification
• Build passes

Related Issues

N/A