Add Docker healthcheck backed by Discord gateway state

[nextinfinity]

Summary

• Add a loopback-only /health endpoint using the JDK HTTP server: HTTP 200 only when the running JDA client reports CONNECTED, otherwise 503.
• Install curl explicitly in the runtime image and use it for the Docker probe. No Java probe helper, bot credentials in probes, or extra Discord sessions.
• Check every 30 seconds with a 5-second Docker timeout, 4-second curl timeout, 60-second startup grace period, and three consecutive failures before unhealthy. Bypass proxies for the local request.
• Document scope and reconnect behavior (including that Docker health status does not itself restart containers).
• Automated tests are deferred to a follow-up holistic test evaluation.

Validation

• ./gradlew shadowJar passes on JDK 25.
• git diff --check passes.
• Docker is unavailable locally, so container execution was not tested. No live Discord token was used.