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/tenancy/services/tenancy_metrics.py

@@ -394,6 +394,84 @@ class TenancyMetricsProvider:
             logger.warning(f"Failed to get tenancy platform metrics: {e}")
             return []
 
+    def get_merchant_metrics(
+        self,
+        db: Session,
+        merchant_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get metrics scoped to a specific merchant.
+
+        Provides:
+        - Total stores owned by this merchant
+        - Active stores
+        - Distinct active team members across all merchant stores
+        """
+        from app.modules.tenancy.models import Store, StoreUser
+
+        try:
+            total_stores = (
+                db.query(Store)
+                .filter(Store.merchant_id == merchant_id)
+                .count()
+            )
+
+            active_stores = (
+                db.query(Store)
+                .filter(
+                    Store.merchant_id == merchant_id,
+                    Store.is_active == True,
+                )
+                .count()
+            )
+
+            # Distinct active team members across all merchant stores
+            merchant_store_ids = (
+                db.query(Store.id)
+                .filter(Store.merchant_id == merchant_id)
+                .subquery()
+            )
+            team_members = (
+                db.query(func.count(func.distinct(StoreUser.user_id)))
+                .filter(
+                    StoreUser.store_id.in_(merchant_store_ids),
+                    StoreUser.is_active == True,
+                )
+                .scalar()
+                or 0
+            )
+
+            return [
+                MetricValue(
+                    key="tenancy.total_stores",
+                    value=total_stores,
+                    label="Total Stores",
+                    category="tenancy",
+                    icon="shopping-bag",
+                    description="Total stores owned by this merchant",
+                ),
+                MetricValue(
+                    key="tenancy.active_stores",
+                    value=active_stores,
+                    label="Active Stores",
+                    category="tenancy",
+                    icon="check-circle",
+                    description="Active stores owned by this merchant",
+                ),
+                MetricValue(
+                    key="tenancy.team_members",
+                    value=team_members,
+                    label="Team Members",
+                    category="tenancy",
+                    icon="users",
+                    description="Distinct active team members across all stores",
+                ),
+            ]
+        except Exception as e:
+            logger.warning(f"Failed to get tenancy merchant metrics: {e}")
+            return []
+
 
 # Singleton instance
 tenancy_metrics_provider = TenancyMetricsProvider()