fix: return 204 No Content from restart endpoints

Empty 200 responses caused JSON parse errors in the API client.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/portal/TenantPortalController.java

@@ -73,7 +73,7 @@ public class TenantPortalController {
     @PostMapping("/server/restart")
     public ResponseEntity<Void> restartServer() {
         portalService.restartServer();
-        return ResponseEntity.ok().build();
+        return ResponseEntity.noContent().build();
     }
 
     @GetMapping("/settings")

src/main/java/net/siegeln/cameleer/saas/vendor/VendorTenantController.java

@@ -113,7 +113,7 @@ public class VendorTenantController {
     public ResponseEntity<Void> restart(@PathVariable UUID id) {
         try {
             vendorTenantService.restartServer(id);
-            return ResponseEntity.ok().build();
+            return ResponseEntity.noContent().build();
         } catch (IllegalArgumentException e) {
             return ResponseEntity.notFound().build();
         }