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/ai-core/cmd/main.go

@@ -547,6 +547,10 @@ func main() {
 			Description string `json:"description"`
 			UserID      string `json:"user_id"`
 			SessionID   string `json:"session_id"`
+			Platform    string `json:"platform,omitempty"`
+			ChannelType string `json:"channel_type,omitempty"`
+			ChannelID   string `json:"channel_id,omitempty"`
+			AdapterName string `json:"adapter_name,omitempty"`
 		}
 		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 			w.WriteHeader(http.StatusBadRequest)
@@ -580,7 +584,22 @@ func main() {
 
 		// Push through the message pusher if available.
 		if thinker != nil {
-			thinker.TriggerReminderMessage(req.UserID, req.SessionID, reminderMsg)
+			// If group reminder, push through platform-bridge via thinker
+			if req.Platform != "" && req.ChannelID != "" {
+				target := background.ProactiveTarget{
+					Platform: req.Platform,
+					ChatType: req.ChannelType,
+					GroupID:  req.ChannelID,
+				}
+				if req.AdapterName != "" {
+					target.Platform = req.AdapterName
+				}
+				thinker.PushPlatformMessage(target, reminderMsg)
+			}
+			// 只有非群聊提醒才推Web端（群聊提醒已通过platform-bridge发送）
+			if req.Platform == "" {
+				thinker.TriggerReminderMessage(req.UserID, req.SessionID, reminderMsg)
+			}
 		}
 		w.Header().Set("Content-Type", "application/json")
 		w.Write([]byte(`{"status":"ok"}`))