fix: use metrics provider pattern for merchant dashboard stats

The merchant dashboard was showing subscription count as "Total Stores".
Add get_merchant_metrics() to MetricsProviderProtocol and implement it
in tenancy, billing, and customer providers. Dashboard now fetches real
stats from a new /merchants/core/dashboard/stats endpoint and displays
4 cards: active subscriptions, total stores, customers, team members.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/billing/services/billing_metrics.py

@@ -108,6 +108,45 @@ class BillingMetricsProvider:
             logger.warning(f"Failed to get billing platform metrics: {e}")
             return []
 
+    def get_merchant_metrics(
+        self,
+        db: Session,
+        merchant_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get subscription metrics for a specific merchant.
+
+        Provides:
+        - Active subscriptions (active + trial)
+        """
+        from app.modules.billing.models import MerchantSubscription
+
+        try:
+            active_subs = (
+                db.query(func.count(MerchantSubscription.id))
+                .filter(
+                    MerchantSubscription.merchant_id == merchant_id,
+                    MerchantSubscription.status.in_(["active", "trial"]),
+                )
+                .scalar()
+                or 0
+            )
+
+            return [
+                MetricValue(
+                    key="billing.active_subscriptions",
+                    value=active_subs,
+                    label="Active Subscriptions",
+                    category="billing",
+                    icon="clipboard-list",
+                    description="Active or trial subscriptions for this merchant",
+                ),
+            ]
+        except Exception as e:
+            logger.warning(f"Failed to get billing merchant metrics: {e}")
+            return []
+
 
 # Singleton instance
 billing_metrics_provider = BillingMetricsProvider()