feat(subscription): 支持调整订阅时长（延长/缩短）

- 将"延长订阅"功能改为"调整订阅"，支持正数延长、负数缩短
- 后端验证：调整天数范围 -36500 到 36500，缩短后剩余天数必须 > 0
- 前端同步更新界面文案和验证逻辑

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/handler/admin/subscription_handler.go

@@ -53,9 +53,9 @@ type BulkAssignSubscriptionRequest struct {
 	Notes        string  `json:"notes"`
 }
 
-// ExtendSubscriptionRequest represents extend subscription request
-type ExtendSubscriptionRequest struct {
-	Days int `json:"days" binding:"required,min=1,max=36500"` // max 100 years
+// AdjustSubscriptionRequest represents adjust subscription request (extend or shorten)
+type AdjustSubscriptionRequest struct {
+	Days int `json:"days" binding:"required,min=-36500,max=36500"` // negative to shorten, positive to extend
 }
 
 // List handles listing all subscriptions with pagination and filters
@@ -180,7 +180,7 @@ func (h *SubscriptionHandler) BulkAssign(c *gin.Context) {
 	response.Success(c, dto.BulkAssignResultFromService(result))
 }
 
-// Extend handles extending a subscription
+// Extend handles adjusting a subscription (extend or shorten)
 // POST /api/v1/admin/subscriptions/:id/extend
 func (h *SubscriptionHandler) Extend(c *gin.Context) {
 	subscriptionID, err := strconv.ParseInt(c.Param("id"), 10, 64)
@@ -189,7 +189,7 @@ func (h *SubscriptionHandler) Extend(c *gin.Context) {
 		return
 	}
 
-	var req ExtendSubscriptionRequest
+	var req AdjustSubscriptionRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		response.BadRequest(c, "Invalid request: "+err.Error())
 		return