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/im/router.go

@@ -0,0 +1,27 @@
+// Package im 提供即时通讯模块
+package im
+
+import (
+	"github.com/echochat/backend/app/im/controller"
+	"github.com/gin-gonic/gin"
+)
+
+// RegisterRoutes 注册 IM 模块的所有路由（需要 JWT 中间件）
+func RegisterRoutes(r *gin.Engine, ctrl *controller.IMController, jwtAuth gin.HandlerFunc) {
+	authed := r.Group("/api/v1/im")
+	authed.Use(jwtAuth)
+	{
+		// 会话管理
+		authed.GET("/conversations", ctrl.GetConversations)
+		authed.PUT("/conversations/:id/pin", ctrl.PinConversation)
+		authed.DELETE("/conversations/:id", ctrl.DeleteConversation)
+		authed.DELETE("/conversations/:id/messages", ctrl.ClearHistory)
+
+		// 消息
+		authed.GET("/messages", ctrl.GetHistoryMessages)
+		authed.GET("/messages/search", ctrl.SearchMessages)
+
+		// 未读数
+		authed.GET("/unread", ctrl.GetTotalUnread)
+	}
+}