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/core/services/stats_aggregator.py

@@ -196,6 +196,68 @@ class StatsAggregatorService:
         categorized = self.get_store_dashboard_stats(db, store_id, platform_id, context)
         return self._flatten_metrics(categorized)
 
+    def get_merchant_dashboard_stats(
+        self,
+        db: Session,
+        merchant_id: int,
+        platform_id: int,
+        context: MetricsContext | None = None,
+    ) -> dict[str, list[MetricValue]]:
+        """
+        Get all metrics for a merchant, grouped by category.
+
+        Called by the merchant dashboard to display merchant-scoped statistics.
+
+        Args:
+            db: Database session
+            merchant_id: ID of the merchant to get metrics for
+            platform_id: Platform ID (for module enablement check)
+            context: Optional filtering/scoping context
+
+        Returns:
+            Dict mapping category name to list of MetricValue objects
+        """
+        providers = self._get_enabled_providers(db, platform_id)
+        result: dict[str, list[MetricValue]] = {}
+
+        for module, provider in providers:
+            if not hasattr(provider, "get_merchant_metrics"):
+                continue
+            try:
+                metrics = provider.get_merchant_metrics(db, merchant_id, context)
+                if metrics:
+                    result[provider.metrics_category] = metrics
+            except Exception as e:
+                logger.warning(
+                    f"Failed to get merchant metrics from module {module.code}: {e}"
+                )
+
+        return result
+
+    def get_merchant_stats_flat(
+        self,
+        db: Session,
+        merchant_id: int,
+        platform_id: int,
+        context: MetricsContext | None = None,
+    ) -> dict[str, Any]:
+        """
+        Get merchant metrics as a flat dictionary.
+
+        Args:
+            db: Database session
+            merchant_id: ID of the merchant to get metrics for
+            platform_id: Platform ID (for module enablement check)
+            context: Optional filtering/scoping context
+
+        Returns:
+            Flat dict mapping metric keys to values
+        """
+        categorized = self.get_merchant_dashboard_stats(
+            db, merchant_id, platform_id, context
+        )
+        return self._flatten_metrics(categorized)
+
     def get_admin_stats_flat(
         self,
         db: Session,