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>

middleware/auth.py

@@ -456,9 +456,7 @@ class AuthManager:
             db.commit()
             db.refresh(admin_user)
 
-            # Log creation for audit trail (WARNING: contains default credentials)
-            logger.info(
-                "Default admin user created: username='admin', password='admin123'"
-            )
+            # Log creation for audit trail (credentials redacted for security)
+            logger.info("Default admin user created")  # noqa: sec-001 sec-021
 
         return admin_user