fix(import): 修复导入 504 超时 + 添加上传进度显示

nginx:
- 新增 /api/v1/import/ 专属 location，proxy_read_timeout 延长至 300s

后端:
- ImportInventory 预加载商品/仓库/库存（3 次 bulk query 替代 N+1）
- 批量写 inventory_log（CreateInBatches 替代逐行 Create）

前端:
- settings 导入行：显示"上传 XX%"进度条和"导入数据（Ns）"计时
- Dio 超时设置为 send=120s / receive=300s

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/screens/settings/settings_screen.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import '../../core/utils/dialog_util.dart';
 import 'package:dio/dio.dart';
 import 'package:file_picker/file_picker.dart';
@@ -1524,10 +1525,22 @@ class _ImportSlot {
   int total = 0;
   int imported = 0;
   int skipped = 0;
+  // 进度
+  int uploadPercent = 0;
+  bool isProcessing = false;
+  int processingSeconds = 0;
 
   _ImportSlot(this.title, this.endpoint, this.hint);
 
   bool get hasResult => success != null;
+
+  void resetProgress() {
+    uploadPercent = 0;
+    isProcessing = false;
+    processingSeconds = 0;
+    success = null;
+    error = null;
+  }
 }
 
 class _BatchImportWidget extends ConsumerStatefulWidget {
@@ -1657,14 +1670,15 @@ class _BatchImportWidgetState extends ConsumerState<_BatchImportWidget> {
     setState(() {
       _loading = true;
       for (final s in _slots) {
-        if (s.file != null) {
-          s.success = null;
-          s.error = null;
-        }
+        if (s.file != null) s.resetProgress();
       }
     });
 
-    final dio = Dio(BaseOptions(baseUrl: AppConfig.apiBaseUrl));
+    final dio = Dio(BaseOptions(
+      baseUrl: AppConfig.apiBaseUrl,
+      sendTimeout: const Duration(seconds: 120),
+      receiveTimeout: const Duration(seconds: 300),
+    ));
     dio.options.headers['Authorization'] = 'Bearer $token';
 
     for (final slot in _slots) {
@@ -1674,29 +1688,56 @@ class _BatchImportWidgetState extends ConsumerState<_BatchImportWidget> {
         if (mounted) setState(() { slot.success = false; slot.error = '无法读取文件'; });
         continue;
       }
+
+      Timer? processingTimer;
+      bool uploadDone = false;
+
       try {
         final formData = FormData.fromMap({
           'file': MultipartFile.fromBytes(bytes, filename: slot.file!.name),
         });
-        final resp = await dio.post(slot.endpoint, data: formData);
+        final resp = await dio.post(
+          slot.endpoint,
+          data: formData,
+          onSendProgress: (sent, total) {
+            if (total <= 0 || !mounted) return;
+            if (sent >= total && !uploadDone) {
+              uploadDone = true;
+              setState(() { slot.isProcessing = true; slot.processingSeconds = 0; });
+              processingTimer = Timer.periodic(const Duration(seconds: 1), (_) {
+                if (!mounted) return;
+                setState(() => slot.processingSeconds++);
+              });
+            } else if (!uploadDone) {
+              final pct = (sent / total * 100).round().clamp(0, 99);
+              if (mounted) setState(() => slot.uploadPercent = pct);
+            }
+          },
+        );
+        processingTimer?.cancel();
         final data = (resp.data is Map) ? resp.data as Map<String, dynamic> : <String, dynamic>{};
         if (mounted) setState(() {
           slot.success = true;
           slot.total = (data['total'] ?? data['imported'] ?? 0) as int;
           slot.imported = (data['imported'] ?? 0) as int;
           slot.skipped = (data['skipped'] ?? 0) as int;
+          slot.isProcessing = false;
         });
       } on DioException catch (e) {
+        processingTimer?.cancel();
         final raw = e.response?.data;
         final msg = (raw is Map ? raw['error'] : null) ?? e.message ?? '未知错误';
         if (mounted) setState(() {
           slot.success = false;
           slot.error = msg.toString();
+          slot.isProcessing = false;
         });
       } catch (e) {
+        processingTimer?.cancel();
         if (mounted) setState(() {
           slot.success = false;
           slot.error = e.toString();
+          slot.isProcessing = false;
         });
       }
     }
@@ -2047,10 +2088,42 @@ class _BatchImportWidgetState extends ConsumerState<_BatchImportWidget> {
         );
       }
     } else if (_loading && slot.file != null) {
-      statusWidget = const SizedBox(
-        width: 14, height: 14,
-        child: CircularProgressIndicator(strokeWidth: 2),
-      );
+      if (slot.isProcessing) {
+        statusWidget = Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            SizedBox(
+              width: 12, height: 12,
+              child: CircularProgressIndicator(strokeWidth: 2, color: AppTheme.primary),
+            ),
+            const SizedBox(width: 6),
+            Text(
+              '导入数据${slot.processingSeconds > 0 ? "（${slot.processingSeconds}秒）" : ""}',
+              style: const TextStyle(fontSize: 13, color: AppTheme.textSecondary),
+            ),
+          ],
+        );
+      } else {
+        statusWidget = Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            SizedBox(
+              width: 80,
+              child: LinearProgressIndicator(
+                value: slot.uploadPercent / 100,
+                backgroundColor: Colors.grey[200],
+                valueColor: AlwaysStoppedAnimation<Color>(AppTheme.primary),
+                minHeight: 6,
+              ),
+            ),
+            const SizedBox(width: 6),
+            Text(
+              '上传 ${slot.uploadPercent}%',
+              style: const TextStyle(fontSize: 12, color: AppTheme.textSecondary),
+            ),
+          ],
+        );
+      }
     }
 
     return Padding(