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)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/admin/__init__.py

@@ -46,6 +46,7 @@ from . import (
     platform_health,
     products,
     settings,
+    subscriptions,
     tests,
     users,
     vendor_domains,
@@ -173,6 +174,14 @@ router.include_router(
 )
 
 
+# ============================================================================
+# Billing & Subscriptions
+# ============================================================================
+
+# Include subscription management endpoints
+router.include_router(subscriptions.router, tags=["admin-subscriptions"])
+
+
 # ============================================================================
 # Code Quality & Architecture
 # ============================================================================