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/services/__init__.py

@@ -0,0 +1,28 @@
+# app/modules/billing/services/__init__.py
+"""
+Billing module services.
+
+Provides subscription management, Stripe integration, and admin operations.
+"""
+
+from app.modules.billing.services.subscription_service import (
+    SubscriptionService,
+    subscription_service,
+)
+from app.modules.billing.services.stripe_service import (
+    StripeService,
+    stripe_service,
+)
+from app.modules.billing.services.admin_subscription_service import (
+    AdminSubscriptionService,
+    admin_subscription_service,
+)
+
+__all__ = [
+    "SubscriptionService",
+    "subscription_service",
+    "StripeService",
+    "stripe_service",
+    "AdminSubscriptionService",
+    "admin_subscription_service",
+]