fix: 统一响应补 hint 字段（对齐 api-contract）

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/common/filters/http-exception.filter.ts

@@ -58,6 +58,7 @@ export class AllExceptionsFilter implements ExceptionFilter {
       code: code ?? status * 100,
       message,
       data: null,
+      hint: null,
     });
   }
 }

src/common/interceptors/transform.interceptor.ts

@@ -20,6 +20,6 @@ function toUnixMs(value: unknown): unknown {
 @Injectable()
 export class TransformInterceptor<T> implements NestInterceptor<T, unknown> {
   intercept(_context: ExecutionContext, next: CallHandler<T>): Observable<unknown> {
-    return next.handle().pipe(map((data) => ({ code: 0, message: 'ok', data: toUnixMs(data ?? null) })));
+    return next.handle().pipe(map((data) => ({ code: 0, message: 'ok', data: toUnixMs(data ?? null), hint: null })));
   }
 }