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/schemas/dashboard.py

@@ -221,6 +221,23 @@ class StoreDashboardStatsResponse(BaseModel):
     revenue: StoreRevenueStats
 
 
+# ============================================================================
+# Merchant Dashboard Statistics
+# ============================================================================
+
+
+class MerchantDashboardStatsResponse(BaseModel):
+    """Merchant dashboard statistics response schema.
+
+    Used by: GET /api/v1/merchants/core/dashboard/stats
+    """
+
+    active_subscriptions: int = Field(0, description="Active or trial subscriptions")
+    total_stores: int = Field(0, description="Total stores owned by this merchant")
+    total_customers: int = Field(0, description="Total customers across all stores")
+    team_members: int = Field(0, description="Distinct active team members across stores")
+
+
 __all__ = [
     # Stats responses
     "StatsResponse",
@@ -240,4 +257,6 @@ __all__ = [
     "StoreRevenueStats",
     "StoreInfo",
     "StoreDashboardStatsResponse",
+    # Merchant dashboard
+    "MerchantDashboardStatsResponse",
 ]