feat(im): Task 5 - IM REST Controller + Router + Wire 集成

IMController (REST API):
- GET /api/v1/im/conversations: 会话列表
- GET /api/v1/im/messages: 历史消息（游标分页）
- PUT /api/v1/im/conversations/:id/pin: 置顶/取消
- DELETE /api/v1/im/conversations/🆔 删除会话
- DELETE /api/v1/im/conversations/:id/messages: 清空记录
- GET /api/v1/im/messages/search: 全局搜索
- GET /api/v1/im/unread: 全局未读数

Wire 集成:
- IMSet Provider Set + wire.Bind 接口注入
- App struct 新增 IM 字段
- wire_gen.go 手动更新
- OfflinePusher 通过 SetOfflinePusher 注入 WS Handler

Made-with: Cursor

backend/go-service/app/provider/wire.go

@@ -7,6 +7,9 @@ import (
 	"github.com/echochat/backend/app/admin"
 	"github.com/echochat/backend/app/auth"
 	"github.com/echochat/backend/app/contact"
+	contactDAO "github.com/echochat/backend/app/contact/dao"
+	imApp "github.com/echochat/backend/app/im"
+	imService "github.com/echochat/backend/app/im/service"
 	wsApp "github.com/echochat/backend/app/ws"
 	"github.com/echochat/backend/config"
 	"github.com/google/wire"
@@ -20,6 +23,9 @@ func InitializeApp(cfg *config.Config) (*App, error) {
 		admin.AdminSet,
 		wsApp.WSSet,
 		contact.ContactSet,
+		imApp.IMSet,
+		wire.Bind(new(imService.FriendChecker), new(*contactDAO.FriendshipDAO)),
+		wire.Bind(new(imService.UserInfoGetter), new(*contactDAO.FriendshipDAO)),
 	)
 	return nil, nil
 }