✨ feat: add subscription billing system with admin management and user purchase flow

Implement a new subscription-based billing model alongside existing metered/per-request billing:

Backend:
- Add subscription plan models (SubscriptionPlan, SubscriptionPlanItem, UserSubscription, etc.)
- Implement CRUD APIs for subscription plan management (admin only)
- Add user subscription queries with support for multiple active/expired subscriptions
- Integrate payment gateways (Stripe, Creem, Epay) for subscription purchases
- Implement pre-consume and post-consume billing logic for subscription quota tracking
- Add billing preference settings (subscription_first, wallet_first, etc.)
- Enhance usage logs with subscription deduction details

Frontend - Admin:
- Add subscription management page with table view and drawer-based edit form
- Match UI/UX style with existing admin pages (redemption codes, users)
- Support enabling/disabling plans, configuring payment IDs, and model quotas
- Add user subscription binding modal in user management

Frontend - Wallet:
- Add subscription plans card with current subscription status display
- Show all subscriptions (active and expired) with remaining days/usage percentage
- Display purchasable plans with pricing cards following SaaS best practices
- Extract purchase modal to separate component matching payment confirm modal style
- Add skeleton loading states with active animation
- Implement billing preference selector in card header
- Handle payment gateway availability based on admin configuration

Frontend - Usage Logs:
- Display subscription deduction details in log entries
- Show step-by-step breakdown of subscription usage (pre-consumed, delta, final, remaining)
- Add subscription deduction tag for subscription-covered requests

model/main.go

@@ -268,6 +268,11 @@ func migrateDB() error {
 		&TwoFA{},
 		&TwoFABackupCode{},
 		&Checkin{},
+		&SubscriptionPlan{},
+		&SubscriptionPlanItem{},
+		&SubscriptionOrder{},
+		&UserSubscription{},
+		&UserSubscriptionItem{},
 	)
 	if err != nil {
 		return err
@@ -302,6 +307,11 @@ func migrateDBFast() error {
 		{&TwoFA{}, "TwoFA"},
 		{&TwoFABackupCode{}, "TwoFABackupCode"},
 		{&Checkin{}, "Checkin"},
+		{&SubscriptionPlan{}, "SubscriptionPlan"},
+		{&SubscriptionPlanItem{}, "SubscriptionPlanItem"},
+		{&SubscriptionOrder{}, "SubscriptionOrder"},
+		{&UserSubscription{}, "UserSubscription"},
+		{&UserSubscriptionItem{}, "UserSubscriptionItem"},
 	}
 	// 动态计算migration数量，确保errChan缓冲区足够大
 	errChan := make(chan error, len(migrations))