新增：媒体上传客户端——MediaUploader 编排状态机 + 预签名直传 + 进度组件，修正 media 端点误挂 community 服务（T3-13）

- MediaUploader：选图→压缩（80→60 降质阶梯，10 MiB 超限拒绝）→createUpload
  →预签名 PUT 直传（进度回调）→confirm→ready assetId 交付；并发槽位 2、
  顺序即 position、单图失败不拖垮整批；凭据过期预检与存储侧 403 各自动
  换新凭据一次；失败重试复用压缩产物、从 createUpload 全新开始
- 孤儿防护：未 confirm 的 assetId 只以管线局部变量存在，快照 assetId 与
  ready 态断言绑定，buildAttachRequests 非全员 ready 即抛 StateError
- 线路修正：media 两步上传端点由 user 服务（:8082 MediaController）提供,
  T3-12 误挂 community 客户端（:8084 无 media 路由），补 mediaApi 分端口
  直连，compose 真链路实测证实
- UploadProgressOverlay 四态进度层（05 号规范 §3.3）；直传层本地
  HttpServer 三线路测试（200/403/断连）；compose 冒烟测试
  （PATBOND_MEDIA_SMOKE=1 启用，默认跳过）
- 新依赖：image_picker、flutter_image_compress（03 号评估选型）
- flutter test 379 全绿（基线 347，+32）、analyze 0、format 无 diff

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

test/helpers/media_test_helpers.dart

@@ -0,0 +1,138 @@
+import 'dart:async';
+import 'dart:typed_data';
+
+import 'package:patbond_flutter/features/community/community_models.dart';
+import 'package:patbond_flutter/features/community/media_compression.dart';
+import 'package:patbond_flutter/features/community/media_direct_upload.dart';
+import 'package:patbond_flutter/features/community/media_picking.dart';
+
+/// media 上传链路测试件（T3-13）：假选择器/压缩器/直传客户端,
+/// 全部支持 Completer 控时序（helpers 既有先例）。
+
+PickedMediaImage pickedImage({int seed = 1, int size = 64}) => PickedMediaImage(
+  bytes: Uint8List.fromList(List.filled(size, seed)),
+  name: 'img-$seed.jpg',
+);
+
+MediaUploadCredentials credentials({
+  String assetId = 'a-1',
+  DateTime? expiresAt,
+}) => MediaUploadCredentials(
+  assetId: assetId,
+  uploadUrl:
+      'http://minio.local/patbond-media/post_image/$assetId?X-Amz-Signature=sig',
+  method: 'PUT',
+  requiredHeaders: const {'Content-Type': 'image/jpeg'},
+  expiresAt: expiresAt ?? DateTime.now().add(const Duration(minutes: 10)),
+);
+
+MediaAsset readyAsset({String assetId = 'a-1'}) => MediaAsset(
+  id: assetId,
+  kind: MediaKind.image,
+  purpose: 'post_image',
+  mimeType: 'image/jpeg',
+  byteSize: 1024,
+  widthPx: 1080,
+  heightPx: 810,
+  status: MediaAssetStatus.ready,
+  url: 'http://minio.local/p.jpg?X-Amz-Signature=sig',
+  readyAt: DateTime.utc(2026, 9, 9),
+  createdAt: DateTime.utc(2026, 9, 9),
+);
+
+class FakeMediaImagePicker implements MediaImagePicker {
+  FakeMediaImagePicker(this.results);
+
+  final List<PickedMediaImage> results;
+  final List<int> limits = [];
+
+  /// 非 null 时 pickImages 挂起等待（picking 态观测用）。
+  Completer<void>? gate;
+
+  @override
+  Future<List<PickedMediaImage>> pickImages({required int limit}) async {
+    limits.add(limit);
+    await gate?.future;
+    return results.take(limit).toList();
+  }
+}
+
+/// 假压缩器：默认原样透传出 image/jpeg；[sizePerQuality] 指定各降质档
+/// 的产物大小（超限阶梯测试用）。
+class FakeMediaCompressor implements MediaImageCompressor {
+  final List<int> qualities = [];
+  Map<int, int>? sizePerQuality;
+  Exception? error;
+
+  @override
+  Future<CompressedMediaImage> compress(
+    PickedMediaImage source, {
+    required int quality,
+  }) async {
+    qualities.add(quality);
+    if (error != null) throw error!;
+    final size = sizePerQuality?[quality];
+    return CompressedMediaImage(
+      bytes: size == null
+          ? source.bytes
+          : Uint8List.fromList(List.filled(size, 0)),
+      mimeType: 'image/jpeg',
+    );
+  }
+}
+
+/// 一次直传调用的观测句柄（进度回调驱动 + 结局手动控制）。
+class DirectUploadCall {
+  DirectUploadCall({
+    required this.url,
+    required this.headers,
+    required this.bytes,
+    required this.onProgress,
+  });
+
+  final String url;
+  final Map<String, String> headers;
+  final Uint8List bytes;
+  final void Function(int sent, int total)? onProgress;
+  final Completer<void> completer = Completer<void>();
+
+  void emitProgress(int sent, int total) => onProgress?.call(sent, total);
+  void succeed() => completer.complete();
+  void fail(MediaDirectUploadException error) => completer.completeError(error);
+}
+
+class FakeDirectUploadClient implements MediaDirectUploadClient {
+  final List<DirectUploadCall> calls = [];
+
+  /// 非 null 时每次 put 自动以该结果收尾（null = 自动成功）；
+  /// 设为 manual 后由测试经 [calls] 手动驱动。
+  bool manual = false;
+  final List<MediaDirectUploadException?> scriptedOutcomes = [];
+
+  @override
+  Future<void> put({
+    required String url,
+    required Map<String, String> headers,
+    required Uint8List bytes,
+    void Function(int sent, int total)? onProgress,
+  }) {
+    final call = DirectUploadCall(
+      url: url,
+      headers: headers,
+      bytes: bytes,
+      onProgress: onProgress,
+    );
+    calls.add(call);
+    if (!manual) {
+      final outcome = scriptedOutcomes.isEmpty
+          ? null
+          : scriptedOutcomes.removeAt(0);
+      if (outcome == null) {
+        call.succeed();
+      } else {
+        call.fail(outcome);
+      }
+    }
+    return call.completer.future;
+  }
+}