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/routes/api/admin_platform_health.py

@@ -172,7 +172,7 @@ async def get_growth_trends(
 
     Returns growth rates and projections for key metrics.
     """
-    from app.modules.billing.services.capacity_forecast_service import (
+    from app.modules.monitoring.services.capacity_forecast_service import (
         capacity_forecast_service,
     )
 
@@ -189,7 +189,7 @@ async def get_scaling_recommendations(
 
     Returns prioritized list of recommendations.
     """
-    from app.modules.billing.services.capacity_forecast_service import (
+    from app.modules.monitoring.services.capacity_forecast_service import (
         capacity_forecast_service,
     )
 
@@ -206,7 +206,7 @@ async def capture_snapshot(
 
     Normally run automatically by daily background job.
     """
-    from app.modules.billing.services.capacity_forecast_service import (
+    from app.modules.monitoring.services.capacity_forecast_service import (
         capacity_forecast_service,
     )