fix(v2): uq_delivery 索引显式升级迁移(AutoMigrate 不改同名索引)+ refund_id NOT NULL DEFAULT ''

AutoMigrate 只按名字判断索引是否存在,P2 时代库升级后同名 uq_delivery 仍停在
2 列,store.EnqueueDelivery 的 3 列 ON CONFLICT 每次调用(含普通 payment
webhook)都报 "ON CONFLICT clause does not match any PRIMARY KEY or UNIQUE
constraint"(reviewer 在 glebarez/sqlite 上实测复现)。新增
model.UpgradeWebhookDeliveryIndex,AutoMigrate 后显式检测/加宽索引 +
backfill refund_id,main.go 与 OpenTestDB 两处调用点接入;RefundID 补
not null;default:'' 补上 NULL/"" 语义缺口。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

internal/model/webhook_delivery.go

@@ -6,7 +6,7 @@ type WebhookDelivery struct {
 	Base
 	OutTradeNo string `gorm:"size:64;not null;uniqueIndex:uq_delivery" json:"out_trade_no"`
 	EventType  string `gorm:"size:32;not null;uniqueIndex:uq_delivery" json:"event_type"`
-	RefundID   string `gorm:"size:64;uniqueIndex:uq_delivery" json:"refund_id,omitempty"` // 退款事件的幂等维度;payment 事件为空
+	RefundID   string `gorm:"size:64;not null;default:'';uniqueIndex:uq_delivery" json:"refund_id,omitempty"` // 退款事件的幂等维度;payment 事件为空
 	BizSystem  string `gorm:"index;size:32" json:"biz_system"`
 	Payload    string `gorm:"type:text" json:"payload"` // 已序列化的领域 JSON(含 event_type)
 	Delivered  bool   `gorm:"index;default:false" json:"delivered"`