refactor(arch): eliminate all cross-module model imports in service layer

Enforce MOD-025/MOD-026 rules: zero top-level cross-module model imports
remain in any service file. All 66 files migrated using deferred import
patterns (method-body, _get_model() helpers, instance-cached self._Model)
and new cross-module service methods in tenancy. Documentation updated
with Pattern 6 (deferred imports), migration plan marked complete, and
violations status reflects 84→0 service-layer violations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/customers/services/customer_metrics.py

@@ -125,18 +125,11 @@ class CustomerMetricsProvider:
         For platforms, aggregates customer data across all stores.
         """
         from app.modules.customers.models import Customer
-        from app.modules.tenancy.models import StorePlatform
+        from app.modules.tenancy.services.platform_service import platform_service
 
         try:
-            # Get all store IDs for this platform using StorePlatform junction table
-            store_ids = (
-                db.query(StorePlatform.store_id)
-                .filter(
-                    StorePlatform.platform_id == platform_id,
-                    StorePlatform.is_active == True,
-                )
-                .subquery()
-            )
+            # Get all store IDs for this platform via platform service
+            store_ids = platform_service.get_store_ids_for_platform(db, platform_id)
 
             # Total customers across all stores
             total_customers = (
@@ -208,14 +201,11 @@ class CustomerMetricsProvider:
         Aggregates customer counts across all stores owned by the merchant.
         """
         from app.modules.customers.models import Customer
-        from app.modules.tenancy.models import Store
+        from app.modules.tenancy.services.store_service import store_service
 
         try:
-            merchant_store_ids = (
-                db.query(Store.id)
-                .filter(Store.merchant_id == merchant_id)
-                .subquery()
-            )
+            merchant_stores = store_service.get_stores_by_merchant_id(db, merchant_id)
+            merchant_store_ids = [s.id for s in merchant_stores]
 
             total_customers = (
                 db.query(Customer)