Feat/mysql checkpointer

[luigisaetta]

Summary

This PR implements the MySQL checkpointer work defined in #288.

It adds MySQL-backed checkpoint persistence for Locus, including:

• MySQLBackend for durable checkpoint storage using MySQL JSON columns
• mysql_checkpointer(...) adapter support for the standard BaseCheckpointer interface
• optional mysql / checkpoints dependency wiring
• integration test coverage gated behind LOCUS_MYSQL_INTEGRATION=1
• Docker Compose setup for local MySQL integration testing
• documentation for running the MySQL integration tests locally

Validation

• Ran the targeted MySQL integration suite against Docker Compose MySQL:
  • TestMySQLBackend
  • TestMySQLAdapter
• Result: 9 passed across Python 3.11, 3.12, 3.13, and 3.14

[luigisaetta]

@fede-kamel
This PR implements the MYSQL Checkpointer.
As explained in the issue (#288) all your suggestions should have been implemented.