Adding a health check that can be used to determine if the worker is healthy

[bra1nDump]

I am using fly.io to deploy and it has health checks, but you can't specify a payload to send, has to be a simple request without any parameters.

This PR

Adds health route (any method) that renders an example mermaid diagram to ensure the worker is responsive.

Related issues in show-me chatgpt plugin project:
bra1nDump/show-me-chatgpt-plugin#42
bra1nDump/show-me-chatgpt-plugin#26

[ggrossetie]

I think we should use /health to be consistent with the gateway server:

kroki/server/src/main/java/io/kroki/server/Server.java

Lines 151 to 158 in 20e7395

	HealthHandler healthHandler = new HealthHandler(registry.getVersions());
	Handler<RoutingContext> healthHandlerService = healthHandler.create();
	router.get("/health")
	.handler(healthHandlerService);
	router.get("/v1/health") // versioned URL (alias)
	.handler(healthHandlerService);
	router.get("/healthz") // k8s liveness default URL (alias)
	.handler(healthHandlerService);

Is fly.io using a GET or HEAD request?

[bra1nDump]

You can specify any http method. Sounds good I will change to /health

[ggrossetie]

Thanks!
All good, running the CI just in case 😅