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/internal/tools/gateway_client.go

@@ -38,7 +38,7 @@ type Reminder struct {
 }
 
 // CreateReminder calls POST /api/v1/internal/reminders on the Gateway.
-func (c *GatewayClient) CreateReminder(ctx context.Context, userID, title, description, remindAt, repeatType, sessionID string) (*Reminder, error) {
+func (c *GatewayClient) CreateReminder(ctx context.Context, userID, title, description, remindAt, repeatType, sessionID, platform, channelType, channelID, adapterName string) (*Reminder, error) {
 	body := map[string]interface{}{
 		"user_id":     userID,
 		"title":       title,
@@ -46,6 +46,10 @@ func (c *GatewayClient) CreateReminder(ctx context.Context, userID, title, descr
 		"remind_at":   remindAt,
 		"repeat_type": repeatType,
 		"session_id":  sessionID,
+		"platform":     platform,
+		"channel_type": channelType,
+		"channel_id":   channelID,
+		"adapter_name": adapterName,
 	}
 	reqBody, _ := json.Marshal(body)