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/billing_metrics.py

@@ -0,0 +1,115 @@
+# app/modules/billing/services/billing_metrics.py
+"""
+Metrics provider for the billing module.
+
+Provides metrics for:
+- Subscription counts (total, active, trial)
+"""
+
+import logging
+
+from sqlalchemy import func
+from sqlalchemy.orm import Session
+
+from app.modules.contracts.metrics import (
+    MetricsContext,
+    MetricValue,
+)
+
+logger = logging.getLogger(__name__)
+
+
+class BillingMetricsProvider:
+    """
+    Metrics provider for billing module.
+
+    Provides subscription metrics at the platform level.
+    """
+
+    @property
+    def metrics_category(self) -> str:
+        return "billing"
+
+    def get_store_metrics(
+        self,
+        db: Session,
+        store_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get metrics for a specific store.
+
+        Subscriptions are merchant-level, not store-level, so no store metrics.
+        """
+        return []
+
+    def get_platform_metrics(
+        self,
+        db: Session,
+        platform_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get subscription metrics aggregated for a platform.
+
+        Provides:
+        - Total subscriptions
+        - Active subscriptions (active + trial)
+        - Trial subscriptions
+        """
+        from app.modules.billing.models import MerchantSubscription, SubscriptionStatus
+
+        try:
+            total_subs = (
+                db.query(func.count(MerchantSubscription.id)).scalar() or 0
+            )
+
+            active_subs = (
+                db.query(func.count(MerchantSubscription.id))
+                .filter(MerchantSubscription.status.in_(["active", "trial"]))
+                .scalar()
+                or 0
+            )
+
+            trial_subs = (
+                db.query(func.count(MerchantSubscription.id))
+                .filter(MerchantSubscription.status == SubscriptionStatus.TRIAL.value)
+                .scalar()
+                or 0
+            )
+
+            return [
+                MetricValue(
+                    key="billing.total_subscriptions",
+                    value=total_subs,
+                    label="Total Subscriptions",
+                    category="billing",
+                    icon="credit-card",
+                    description="Total number of merchant subscriptions",
+                ),
+                MetricValue(
+                    key="billing.active_subscriptions",
+                    value=active_subs,
+                    label="Active Subscriptions",
+                    category="billing",
+                    icon="check-circle",
+                    description="Subscriptions with active or trial status",
+                ),
+                MetricValue(
+                    key="billing.trial_subscriptions",
+                    value=trial_subs,
+                    label="Trial Subscriptions",
+                    category="billing",
+                    icon="clock",
+                    description="Subscriptions currently in trial period",
+                ),
+            ]
+        except Exception as e:
+            logger.warning(f"Failed to get billing platform metrics: {e}")
+            return []
+
+
+# Singleton instance
+billing_metrics_provider = BillingMetricsProvider()
+
+__all__ = ["BillingMetricsProvider", "billing_metrics_provider"]