task-67: 复用 RustFS/MinIO 客户端与 HTTP 连接池

- buildClient 改为 double-check 懒加载缓存共享 MinioClient（volatile 单例），
  同一实例持有同一 OkHttpClient，连接池随实例共享，不再每次操作新建客户端
- supplier 注入路径优先且不写共享缓存，测试隔离不受污染
- 未配置/空白 endpoint 时不创建实例，拒绝操作并保持缓存为空

backend-java/src/main/java/com/nanri/aiimage/modules/file/service/object/RustfsObjectStorageService.java

@@ -49,6 +49,11 @@ public class RustfsObjectStorageService {
     private volatile long failureWindowStartedAtMillis;
     private volatile long circuitOpenUntilMillis;
     private volatile OkHttpClient httpClient;
+    /**
+     * 懒加载缓存的共享 MinioClient（double-check 单例）：复用同一实例与同一
+     * OkHttpClient（连接池随实例共享），避免每次操作创建客户端。
+     */
+    private volatile MinioClient sharedMinioClient;
 
     @Autowired
     public RustfsObjectStorageService(TransientStorageProperties properties,
@@ -275,12 +280,21 @@ public class RustfsObjectStorageService {
         if (minioClientSupplier != null) {
             return minioClientSupplier.get();
         }
-        return MinioClient.builder()
-                .endpoint(properties.getEndpoint())
-                .credentials(properties.getAccessKeyId(), properties.getAccessKeySecret())
-                .region(properties.getRegion())
-                .httpClient(getHttpClient(deadlineNanos))
-                .build();
+        MinioClient shared = sharedMinioClient;
+        if (shared != null) {
+            return shared;
+        }
+        synchronized (this) {
+            if (sharedMinioClient == null) {
+                sharedMinioClient = MinioClient.builder()
+                        .endpoint(properties.getEndpoint())
+                        .credentials(properties.getAccessKeyId(), properties.getAccessKeySecret())
+                        .region(properties.getRegion())
+                        .httpClient(getHttpClient(deadlineNanos))
+                        .build();
+            }
+            return sharedMinioClient;
+        }
     }
 
     OkHttpClient getHttpClient() {