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/routes/api/merchant.py

@@ -0,0 +1,22 @@
+# app/modules/core/routes/api/merchant.py
+"""
+Core module merchant API routes.
+
+Auto-discovered by the route system (merchant.py in routes/api/ triggers
+registration under /api/v1/merchants/core/*).
+
+Aggregates:
+- /dashboard/* - Merchant dashboard statistics
+"""
+
+from fastapi import APIRouter
+
+from .merchant_dashboard import merchant_dashboard_router
+
+ROUTE_CONFIG = {
+    "prefix": "/core",
+}
+
+router = APIRouter()
+
+router.include_router(merchant_dashboard_router, tags=["merchant-dashboard"])

app/modules/core/routes/api/merchant_dashboard.py

@@ -0,0 +1,58 @@
+# app/modules/core/routes/api/merchant_dashboard.py
+"""
+Merchant dashboard statistics endpoint.
+
+Merchant Context: Uses get_merchant_for_current_user to resolve the merchant
+from the JWT token. The dependency guarantees an active merchant exists.
+
+This module uses the StatsAggregator service from core to collect metrics from all
+enabled modules via the MetricsProvider protocol.
+"""
+
+import logging
+
+from fastapi import APIRouter, Depends, Request
+from sqlalchemy.orm import Session
+
+from app.api.deps import get_merchant_for_current_user
+from app.core.database import get_db
+from app.modules.core.schemas.dashboard import MerchantDashboardStatsResponse
+from app.modules.core.services.stats_aggregator import stats_aggregator
+
+merchant_dashboard_router = APIRouter(prefix="/dashboard")
+logger = logging.getLogger(__name__)
+
+
+@merchant_dashboard_router.get("/stats", response_model=MerchantDashboardStatsResponse)
+def get_merchant_dashboard_stats(
+    request: Request,
+    merchant=Depends(get_merchant_for_current_user),
+    db: Session = Depends(get_db),
+):
+    """
+    Get merchant-scoped dashboard statistics.
+
+    Returns aggregated statistics for the current merchant:
+    - Active subscriptions
+    - Total stores
+    - Total customers across all stores
+    - Team members across all stores
+
+    Merchant is resolved from the JWT token.
+    Requires Authorization header (API endpoint).
+    """
+    platform = getattr(request.state, "platform", None)
+    platform_id = platform.id if platform else 1
+
+    flat = stats_aggregator.get_merchant_stats_flat(
+        db=db,
+        merchant_id=merchant.id,
+        platform_id=platform_id,
+    )
+
+    return MerchantDashboardStatsResponse(
+        active_subscriptions=int(flat.get("billing.active_subscriptions", 0)),
+        total_stores=int(flat.get("tenancy.total_stores", 0)),
+        total_customers=int(flat.get("customers.total", 0)),
+        team_members=int(flat.get("tenancy.team_members", 0)),
+    )