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/models/__init__.py

@@ -0,0 +1,52 @@
+# app/modules/billing/models/__init__.py
+"""
+Billing module models.
+
+Re-exports subscription models from the central models location.
+Models remain in models/database/ for now to avoid breaking existing imports
+across the codebase. This provides a module-local import path.
+
+Usage:
+    from app.modules.billing.models import (
+        VendorSubscription,
+        SubscriptionTier,
+        SubscriptionStatus,
+        TierCode,
+    )
+"""
+
+from models.database.subscription import (
+    # Enums
+    TierCode,
+    SubscriptionStatus,
+    AddOnCategory,
+    BillingPeriod,
+    # Models
+    SubscriptionTier,
+    AddOnProduct,
+    VendorAddOn,
+    StripeWebhookEvent,
+    BillingHistory,
+    VendorSubscription,
+    CapacitySnapshot,
+    # Legacy constants
+    TIER_LIMITS,
+)
+
+__all__ = [
+    # Enums
+    "TierCode",
+    "SubscriptionStatus",
+    "AddOnCategory",
+    "BillingPeriod",
+    # Models
+    "SubscriptionTier",
+    "AddOnProduct",
+    "VendorAddOn",
+    "StripeWebhookEvent",
+    "BillingHistory",
+    "VendorSubscription",
+    "CapacitySnapshot",
+    # Legacy constants
+    "TIER_LIMITS",
+]