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/customers/services/customer_metrics.py

@@ -196,6 +196,47 @@ class CustomerMetricsProvider:
             logger.warning(f"Failed to get customer platform metrics: {e}")
             return []
 
+    def get_merchant_metrics(
+        self,
+        db: Session,
+        merchant_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get customer metrics scoped to a merchant.
+
+        Aggregates customer counts across all stores owned by the merchant.
+        """
+        from app.modules.customers.models import Customer
+        from app.modules.tenancy.models import Store
+
+        try:
+            merchant_store_ids = (
+                db.query(Store.id)
+                .filter(Store.merchant_id == merchant_id)
+                .subquery()
+            )
+
+            total_customers = (
+                db.query(Customer)
+                .filter(Customer.store_id.in_(merchant_store_ids))
+                .count()
+            )
+
+            return [
+                MetricValue(
+                    key="customers.total",
+                    value=total_customers,
+                    label="Total Customers",
+                    category="customers",
+                    icon="users",
+                    description="Total customers across all merchant stores",
+                ),
+            ]
+        except Exception as e:
+            logger.warning(f"Failed to get customer merchant metrics: {e}")
+            return []
+
 
 # Singleton instance
 customer_metrics_provider = CustomerMetricsProvider()