Replace manual health endpoint with Spring Boot Actuator

Remove the hand-rolled HealthController and use spring-boot-starter-actuator
instead. Only the health endpoint is exposed, with no detail leakage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/main/java/de/fete/adapter/in/web/HealthController.java

@@ -1,16 +0,0 @@
-package de.fete.adapter.in.web;
-
-import java.util.Map;
-import org.springframework.web.bind.annotation.GetMapping;
-import org.springframework.web.bind.annotation.RestController;
-
-/** REST endpoint for health checks. */
-@RestController
-public class HealthController {
-
-  /** Returns a simple health status. */
-  @GetMapping("/health")
-  public Map<String, String> health() {
-    return Map.of("status", "ok");
-  }
-}

backend/src/main/resources/application.properties

@@ -1 +1,4 @@
 spring.application.name=fete
+
+management.endpoints.web.exposure.include=health
+management.endpoint.health.show-details=never