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/contracts/metrics.py

@@ -207,6 +207,28 @@ class MetricsProviderProtocol(Protocol):
         """
         ...
 
+    def get_merchant_metrics(
+        self,
+        db: "Session",
+        merchant_id: int,
+        context: MetricsContext | None = None,
+    ) -> list[MetricValue]:
+        """
+        Get metrics scoped to a specific merchant.
+
+        Called by the merchant dashboard to display merchant-scoped statistics.
+        Should only include data belonging to the specified merchant.
+
+        Args:
+            db: Database session for queries
+            merchant_id: ID of the merchant to get metrics for
+            context: Optional filtering/scoping context
+
+        Returns:
+            List of MetricValue objects for this merchant
+        """
+        ...
+
 
 __all__ = [
     "MetricValue",