提交2: 发布群核心功能实现

- 实现 EnsureListingConversation: 发布时建发布群,发欢迎语+二维码
- 实现 AddRenterToListingConversation: 付款后拉租客进群
- 实现 RemoveRenterFromListingConversation: 订单终态移出租客
- 实现消息可见性过滤: 租客只能看到 joined_at 之后的消息
- listing 模块注入 ListingChatCreator 接口,发布时建群
- ListingDTO 新增 listing_group_conversation_id 字段
- 付款流程改为拉租客进发布群(替代建订单群)
- 订单完成和取消时自动移出租客
- 创建 chat 适配器实现接口解耦

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/modules/listing/repository.go

@@ -6,16 +6,26 @@ import (
 	"time"
 
 	"gorm.io/gorm"
+	"hfb_sys/backend/internal/model"
 )
 
+// ListingChatCreator 发布群创建接口(由 chat 模块实现,避免直接依赖)
+type ListingChatCreator interface {
+	EnsureListingConversation(tx *gorm.DB, listing model.RentalListing) (conversationID uint64, err error)
+}
+
 type Repository struct {
 	db                 *gorm.DB
 	publicZoneCountsMu sync.Mutex
 	publicZoneCounts   publicZoneCountCache
+	chatCreator        ListingChatCreator
 }
 
-func NewRepository(db *gorm.DB) *Repository {
-	return &Repository{db: db}
+func NewRepository(db *gorm.DB, chatCreator ListingChatCreator) *Repository {
+	return &Repository{
+		db:          db,
+		chatCreator: chatCreator,
+	}
 }
 
 type publicZoneCountCache struct {