feat(client): 慢请求监控（>500ms 记日志+上报）；拆除列表 provider 的 _cache 失败兜底

- SlowRequestInterceptor：单次 API 调用超 500ms 即 debugPrint long-request
  日志并经 ErrorReporter 上报（error_type=slow_api，按 method+归一路径 5 分钟
  节流）。服务端 GIN 日志只见自身处理耗时，网络往返段只有客户端可观测。
- 拆除 10 个列表 provider 的 _cache 失败兜底：实测线上接口客户端视角典型
  50-120ms、最坏约 270ms（列表类全部 <500ms），失败静默端旧数据弊大于利
  （曾放大跨账号残留问题），改为明确进入错误态由用户重试。

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

client/lib/providers/inventory_provider.dart

@@ -35,20 +35,12 @@ class InventoryListNotifier extends AsyncNotifier<PageResult<Inventory>> {
   bool _sortAsc = true;
   List<String> _series = [];
   List<String> _spec = [];
-  PageResult<Inventory>? _cache;
 
   @override
   Future<PageResult<Inventory>> build() async {
     ref.watch(authStateProvider.select((s) => s.user?.shopId));
     ref.watch(networkRecoveryCountProvider); // 网络恢复时自动刷新
-    try {
-      final result = await _fetch();
-      _cache = result;
-      return result;
-    } catch (_) {
-      if (_cache != null) return _cache!;
-      rethrow;
-    }
+    return _fetch();
   }
 
   Future<PageResult<Inventory>> _fetch() {
@@ -125,14 +117,9 @@ class InventoryListNotifier extends AsyncNotifier<PageResult<Inventory>> {
     state = const AsyncValue<PageResult<Inventory>>.loading()
         .copyWithPrevious(state);
     _fetch().then((result) {
-      _cache = result;
       state = AsyncValue.data(result);
     }, onError: (e, st) {
-      if (_cache != null) {
-        state = AsyncValue.data(_cache!);
-      } else {
-        state = AsyncValue.error(e, st);
-      }
+      state = AsyncValue.error(e, st);
     });
   }
 }
@@ -145,20 +132,12 @@ final inventoryLogProvider =
 class InventoryLogNotifier extends AsyncNotifier<PageResult<InventoryLog>> {
   int _page = 1;
   int _pageSize = AppConstants.defaultPageSize;
-  PageResult<InventoryLog>? _cache;
 
   @override
   Future<PageResult<InventoryLog>> build() async {
     ref.watch(authStateProvider.select((s) => s.user?.shopId));
     ref.watch(networkRecoveryCountProvider);
-    try {
-      final result = await _fetch();
-      _cache = result;
-      return result;
-    } catch (_) {
-      if (_cache != null) return _cache!;
-      rethrow;
-    }
+    return _fetch();
   }
 
   Future<PageResult<InventoryLog>> _fetch() {
@@ -182,14 +161,9 @@ class InventoryLogNotifier extends AsyncNotifier<PageResult<InventoryLog>> {
   void reload() {
     state = const AsyncValue.loading();
     _fetch().then((result) {
-      _cache = result;
       state = AsyncValue.data(result);
     }, onError: (e, st) {
-      if (_cache != null) {
-        state = AsyncValue.data(_cache!);
-      } else {
-        state = AsyncValue.error(e, st);
-      }
+      state = AsyncValue.error(e, st);
     });
   }
 }