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/pom.xml

@@ -27,6 +27,11 @@
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
 
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-actuator</artifactId>
+        </dependency>
+
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>