feat: report pod address from echo endpoint

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/main/java/com/example/keycloakpattern/ApiController.java

@@ -43,6 +43,9 @@ public class ApiController {
         Map<String, Object> response = new LinkedHashMap<>();
         response.put("headers", headers);
         response.put("remoteAddr", request.getRemoteAddr());
+        // Pod IP. Identifies which replica answered, which is what makes the
+        // host nginx upstream distribution and the sticky-session switch observable.
+        response.put("localAddr", request.getLocalAddr());
         response.put("scheme", request.getScheme());
         response.put("secure", request.isSecure());
         response.put("serverName", request.getServerName());