feat: complete billing module migration (Phase 5)

Migrates billing module to self-contained structure:
- Create app/modules/billing/services/ with subscription, stripe, admin services
- Create app/modules/billing/models/ re-exporting from central location
- Create app/modules/billing/schemas/ re-exporting from central location
- Create app/modules/billing/tasks/ with 4 scheduled Celery tasks
- Create app/modules/billing/exceptions.py with module-specific exceptions
- Update definition.py with is_self_contained=True and scheduled_tasks

Celery task migration:
- reset_period_counters -> billing module
- check_trial_expirations -> billing module
- sync_stripe_status -> billing module
- cleanup_stale_subscriptions -> billing module
- capture_capacity_snapshot remains in legacy (will go to monitoring)

Backward compatibility:
- Create re-exports in app/services/ for subscription, stripe, admin services
- Old import paths continue to work
- Update celery_config.py to use module-defined schedules

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

app/modules/billing/__init__.py

@@ -7,6 +7,7 @@ This module provides:
 - Vendor subscription CRUD
 - Billing history and invoices
 - Stripe integration
+- Scheduled tasks for subscription lifecycle
 
 Routes:
 - Admin: /api/v1/admin/subscriptions/*
@@ -15,8 +16,17 @@ Routes:
 Menu Items:
 - Admin: subscription-tiers, subscriptions, billing-history
 - Vendor: billing, invoices
+
+Usage:
+    from app.modules.billing import billing_module
+    from app.modules.billing.services import subscription_service, stripe_service
+    from app.modules.billing.models import VendorSubscription, SubscriptionTier
+    from app.modules.billing.exceptions import TierLimitExceededException
 """
 
-from app.modules.billing.definition import billing_module
+from app.modules.billing.definition import billing_module, get_billing_module_with_routers
 
-__all__ = ["billing_module"]
+__all__ = [
+    "billing_module",
+    "get_billing_module_with_routers",
+]