feat: 群聊提醒 — 提醒关联平台/频道/适配器，到期发回原群聊

- reminder_create 新增 platform/channel_type/channel_id/adapter_name 参数
- Gateway Reminder 模型 + DB 迁移 + CRUD 全部支持新字段
- 提醒到期时：群聊提醒 → platform-bridge 发回原群聊
- 非群聊提醒 → 走 Web 端推送
- Thinker 新增 PushPlatformMessage 方法

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

backend/gateway/internal/handler/reminder_handler.go

@@ -35,7 +35,11 @@ type CreateReminderRequest struct {
 	RemindAt    string `json:"remind_at" binding:"required"` // ISO 8601 格式
 	RepeatType  string `json:"repeat_type"`                  // none, daily, weekly, monthly
 	SessionID   string `json:"session_id"`
-	UserID      string `json:"user_id,omitempty"` // 内部调用时直接传 userID
+	UserID      string `json:"user_id,omitempty"`
+	Platform    string `json:"platform,omitempty"`
+	ChannelType string `json:"channel_type,omitempty"`
+	ChannelID   string `json:"channel_id,omitempty"`
+	AdapterName string `json:"adapter_name,omitempty"` // 内部调用时直接传 userID
 }
 
 // UpdateReminderRequest 更新提醒请求体
@@ -129,6 +133,10 @@ func (h *ReminderHandler) Create(c *gin.Context) {
 		Status:      "pending",
 		RepeatType:  repeatType,
 		SessionID:   req.SessionID,
+		Platform:    req.Platform,
+		ChannelType: req.ChannelType,
+		ChannelID:   req.ChannelID,
+		AdapterName: req.AdapterName,
 		Notified:    false,
 	}
 
@@ -356,10 +364,14 @@ func calculateNextRemindAt(current time.Time, repeatType string) time.Time {
 
 func triggerLLMReminder(aiCoreURL, internalToken string, r store.Reminder) {
 	body := map[string]string{
-		"title":       r.Title,
-		"description": r.Description,
-		"user_id":     r.UserID,
-		"session_id":  r.SessionID,
+		"title":        r.Title,
+		"description":  r.Description,
+		"user_id":      r.UserID,
+		"session_id":   r.SessionID,
+		"platform":     r.Platform,
+		"channel_type": r.ChannelType,
+		"channel_id":   r.ChannelID,
+		"adapter_name": r.AdapterName,
 	}
 	reqBody, _ := json.Marshal(body)
 	req, err := http.NewRequest("POST", aiCoreURL+"/api/v1/internal/reminder-trigger", bytes.NewReader(reqBody))