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/services/__init__.py

@@ -21,10 +21,6 @@ from app.modules.billing.services.billing_service import (
     BillingService,
     billing_service,
 )
-from app.modules.billing.services.capacity_forecast_service import (
-    CapacityForecastService,
-    capacity_forecast_service,
-)
 from app.modules.billing.services.feature_service import (
     FeatureService,
     feature_service,
@@ -68,8 +64,6 @@ __all__ = [
     "SubscriptionNotCancelledError",
     "FeatureService",
     "feature_service",
-    "CapacityForecastService",
-    "capacity_forecast_service",
     "PlatformPricingService",
     "platform_pricing_service",
     "UsageService",