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/templates/admin/partials/sidebar.html

@@ -93,6 +93,14 @@
         {{ menu_item('marketplace-letzshop', '/admin/marketplace/letzshop', 'shopping-cart', 'Letzshop') }}
     {% endcall %}
 
+    <!-- Billing & Subscriptions Section -->
+    {{ section_header('Billing & Subscriptions', 'billing') }}
+    {% call section_content('billing') %}
+        {{ menu_item('subscription-tiers', '/admin/subscription-tiers', 'tag', 'Subscription Tiers') }}
+        {{ menu_item('subscriptions', '/admin/subscriptions', 'credit-card', 'Vendor Subscriptions') }}
+        {{ menu_item('billing-history', '/admin/billing-history', 'document-text', 'Billing History') }}
+    {% endcall %}
+
     <!-- Content Management Section -->
     {{ section_header('Content Management', 'contentMgmt') }}
     {% call section_content('contentMgmt') %}