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/internal/model/error_report.go

@@ -0,0 +1,20 @@
+package model
+
+import "time"
+
+// ErrorReport 客户端错误上报记录（系统级，不做多租户隔离）
+type ErrorReport struct {
+	ID         uint64    `gorm:"primaryKey;autoIncrement" json:"id"`
+	CreatedAt  time.Time `json:"created_at"`
+	ErrorType  string    `gorm:"size:30;not null;index"  json:"error_type"`  // flutter_error / zone_error / caught_exception
+	AppVersion string    `gorm:"size:30"                 json:"app_version"`
+	Platform   string    `gorm:"size:20"                 json:"platform"`    // ios/android/web/windows/macos
+	Username   string    `gorm:"size:50;index"           json:"username"`
+	ShopID     uint64    `gorm:"index"                   json:"shop_id"`
+	ShopNo     string    `gorm:"size:50"                 json:"shop_no"`
+	Role       string    `gorm:"size:20"                 json:"role"`
+	ClientIP   string    `gorm:"size:60"                 json:"client_ip"`
+	ErrorMsg   string    `gorm:"type:text;not null"      json:"error_msg"`
+	StackTrace string    `gorm:"type:text"               json:"stack_trace"`
+	OccurredAt time.Time `json:"occurred_at"`
+}