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/services/__init__.py

@@ -9,6 +9,10 @@ from app.modules.monitoring.services.admin_audit_service import (
     AdminAuditService,
     admin_audit_service,
 )
+from app.modules.monitoring.services.capacity_forecast_service import (
+    CapacityForecastService,
+    capacity_forecast_service,
+)
 from app.modules.monitoring.services.background_tasks_service import (
     BackgroundTasksService,
     background_tasks_service,
@@ -25,6 +29,8 @@ from app.modules.monitoring.services.platform_health_service import (
 __all__ = [
     "admin_audit_service",
     "AdminAuditService",
+    "capacity_forecast_service",
+    "CapacityForecastService",
     "background_tasks_service",
     "BackgroundTasksService",
     "log_service",