refactor: move capacity_forecast_service from billing to monitoring

Resolves the billing (core) → monitoring (optional) architecture violation
by moving CapacityForecastService to the monitoring module where it belongs.

- Create BillingMetricsProvider to expose subscription counts via stats_aggregator
- Move CapacitySnapshot model from billing to monitoring
- Replace direct MerchantSubscription queries with stats_aggregator calls
- Fix middleware test mocks to cover StoreDomain/MerchantDomain fallback chains

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/billing/models/__init__.py

@@ -22,7 +22,6 @@ from app.modules.billing.models.subscription import (
     AddOnProduct,
     BillingHistory,
     BillingPeriod,
-    CapacitySnapshot,
     StoreAddOn,
     StripeWebhookEvent,
     SubscriptionStatus,
@@ -46,7 +45,6 @@ __all__ = [
     "StoreAddOn",
     "StripeWebhookEvent",
     "BillingHistory",
-    "CapacitySnapshot",
     # Merchant Subscription
     "MerchantSubscription",
     # Feature Limits