feat: 错误自动上报机制

- 后端：error_reports 表 + POST /api/v1/public/errors + GET /api/v1/admin/errors
- Flutter：ErrorReporter 单例，60s 去重，fire-and-forget，不影响用户
- main.dart：FlutterError 和 Zone 错误自动上报
- 打印异常：catch 块加 reportError，Windows 打印问题可自动采集堆栈

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

backend/main.go

@@ -104,6 +104,7 @@ func autoMigrate(db *gorm.DB) {
 		&model.ProductSeriesOption{},
 		&model.ProductSpecOption{},
 		&model.ProductImage{},
+		&model.ErrorReport{},
 	)
 	if err != nil {
 		log.Fatalf("auto migrate failed: %v", err)