feat: support MySQL primary database

[arena-ai-coding-agent]

Summary

• add MySQL as a supported primary database driver and migration target
• add consolidated MySQL schema migrations plus mysql docker-compose profiles
• make dialect-specific repository SQL work across PostgreSQL, MySQL, and SQLite where needed
• document MySQL deployment and migration usage

Validation

• git diff --check

Note: Go and Docker CLIs are not available in this sandbox, so compile/tests and live MySQL migration validation could not be run here.

[arena-ai-coding-agent]

根据要求关闭：本次任务应向 Tencent/WeKnora 发起 PR，不合并到个人 fork 的 main 分支。