fix(config): structured 400 body on unknown apply value

Replace empty-body ResponseEntity.status(BAD_REQUEST).build() with
ResponseStatusException so Spring returns the usual error body shape
with a descriptive reason string, matching the idiom used by
UserAdminController, AppSettingsController, ThresholdAdminController.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cameleer-server-app/src/main/java/com/cameleer/server/app/controller/ApplicationConfigController.java

@@ -34,6 +34,7 @@ import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.security.core.Authentication;
 import org.springframework.web.bind.annotation.*;
+import org.springframework.web.server.ResponseStatusException;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -122,7 +123,8 @@ public class ApplicationConfigController {
                                                               Authentication auth,
                                                               HttpServletRequest httpRequest) {
         if (!"staged".equalsIgnoreCase(apply) && !"live".equalsIgnoreCase(apply)) {
-            return ResponseEntity.status(HttpStatus.BAD_REQUEST).build();
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST,
+                    "Unknown apply value '" + apply + "' — must be 'staged' or 'live'");
         }
 
         String updatedBy = auth != null ? auth.getName() : "system";