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/test/java/de/fete/FeteApplicationTest.java

@@ -24,8 +24,8 @@ class FeteApplicationTest {
 
   @Test
   void healthEndpointReturns200() throws Exception {
-    mockMvc.perform(get("/health"))
+    mockMvc.perform(get("/actuator/health"))
         .andExpect(status().isOk())
-        .andExpect(jsonPath("$.status").value("ok"));
+        .andExpect(jsonPath("$.status").value("UP"));
   }
 }