feat(api): add health route in axum api

[Akinator31]

This pull request introduces a new /api/health endpoint to the API, allowing external systems to check the health status of the bot and its database connection. The handler performs checks on the database, bot running state, connectivity, and latency, returning a detailed JSON status. The new handler is integrated into the API router and made available for use throughout the codebase.

New Health Endpoint:

• Added handle_health function in crates/rustmail/src/api/handler/health.rs to check database connectivity, bot running state, connection status, and latency, and return a structured JSON health report.
• Registered the new /api/health route in the API router (crates/rustmail/src/api/router.rs) to expose the health check endpoint.

Module Integration:

• Added health module to the handler modules (crates/rustmail/src/api/handler/mod.rs).
• Re-exported the health handler for use in other parts of the application (crates/rustmail/src/api/handler/mod.rs).