Implement 5 subscription endpoints matching frontend subscription.ts:
API Endpoints:
- GET /api/v1/subscription/current — get current subscription info
- GET /api/v1/subscription/billing-records — get billing history
- POST /api/v1/subscription/change-plan — upgrade/downgrade plan
- POST /api/v1/subscription/cancel — cancel subscription
- POST /api/v1/subscription/toggle-auto-renew — toggle auto-renewal
Implementation Details:
- Created subscription schemas (SubscriptionInfo, BillingRecord, etc.)
- Created subscription routes with proper authentication
- Integrated with quota_registry for plan limits
- Added plan validation (free/standard/pro/enterprise)
- Added billing cycle validation (monthly/yearly)
- Registered subscription router in api_router
Note: Billing records currently returns empty list (TODO: implement billing system)
Auto-renew toggle is simulated (TODO: add auto_renew field to User model)
All endpoints follow existing patterns from titles/voices/duplication APIs.