feat: add admin frontend for subscription and billing management
Add admin pages for managing subscription tiers, vendor subscriptions,
and billing history:
- Subscription Tiers page: Create, edit, activate/deactivate tiers
- Vendor Subscriptions page: View/edit subscriptions, custom limits
- Billing History page: View invoices with filters and PDF links
- Stats dashboard with MRR/ARR calculations
Also includes:
- Pydantic schemas for billing operations (models/schema/billing.py)
- Admin subscription service layer for database operations
- Security validation fixes (SEC-001, SEC-021, SEC-022)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Samir Boulahtit
d2daf34c90