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/definition.py

@@ -89,6 +89,13 @@ def _get_store_router():
     return store_router
 
 
+def _get_metrics_provider():
+    """Lazy import of metrics provider to avoid circular imports."""
+    from app.modules.billing.services.billing_metrics import billing_metrics_provider
+
+    return billing_metrics_provider
+
+
 def _get_feature_provider():
     """Lazy import of feature provider to avoid circular imports."""
     from app.modules.billing.services.billing_features import billing_feature_provider
@@ -271,6 +278,8 @@ billing_module = ModuleDefinition(
     ],
     # Feature provider for feature flags
     feature_provider=_get_feature_provider,
+    # Metrics provider for subscription metrics
+    metrics_provider=_get_metrics_provider,
 )