fix: internal reminder路由+user_id支持 — 修复401/404

- Gateway新增 POST/GET/DELETE /api/v1/internal/reminders
- CreateReminderRequest新增UserID字段，JWT为空时回退
- GatewayClient URL改为 /api/v1/internal/reminders

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

backend/gateway/internal/handler/reminder_handler.go

@@ -35,6 +35,7 @@ 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
 }
 
 // UpdateReminderRequest 更新提醒请求体
@@ -95,6 +96,12 @@ func (h *ReminderHandler) Create(c *gin.Context) {
 
 	// 从 JWT 获取 userID
 	userID := middleware.GetUserID(c)
+	if userID == "" {
+		userID = req.UserID
+	}
+	if userID == "" {
+		userID = req.UserID
+	}
 	if userID == "" {
 		c.JSON(http.StatusUnauthorized, gin.H{"error": "未认证"})
 		return