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/monitoring/models/__init__.py

@@ -2,11 +2,10 @@
 """
 Monitoring module database models.
 
-Re-exports monitoring-related models from their source locations.
+Provides monitoring-related models including capacity snapshots.
 """
 
-# CapacitySnapshot is in billing module (tracks system capacity over time)
-from app.modules.billing.models import CapacitySnapshot
+from app.modules.monitoring.models.capacity_snapshot import CapacitySnapshot
 
 # Admin notification and logging models
 from app.modules.messaging.models import AdminNotification