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
