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/messaging/services/email_service.py

@@ -369,11 +369,10 @@ def get_platform_email_config(db: Session) -> dict:
     Returns:
         Dictionary with all email configuration values
     """
-    from app.modules.tenancy.models import AdminSetting
+    from app.modules.core.services.admin_settings_service import admin_settings_service
 
     def get_db_setting(key: str) -> str | None:
-        setting = db.query(AdminSetting).filter(AdminSetting.key == key).first()
-        return setting.value if setting else None
+        return admin_settings_service.get_setting_value(db, key)
 
     config = {}
 
@@ -999,10 +998,10 @@ class EmailService:
     def _get_store(self, store_id: int):
         """Get store with caching."""
         if store_id not in self._store_cache:
-            from app.modules.tenancy.models import Store
+            from app.modules.tenancy.services.store_service import store_service
 
-            self._store_cache[store_id] = (
-                self.db.query(Store).filter(Store.id == store_id).first()
+            self._store_cache[store_id] = store_service.get_store_by_id_optional(
+                self.db, store_id
             )
         return self._store_cache[store_id]
 
@@ -1121,11 +1120,9 @@ class EmailService:
 
         # 2. Customer's preferred language
         if customer_id:
-            from app.modules.customers.models.customer import Customer
+            from app.modules.customers.services.customer_service import customer_service
 
-            customer = (
-                self.db.query(Customer).filter(Customer.id == customer_id).first()
-            )
+            customer = customer_service.get_customer_by_id(self.db, customer_id)
             if customer and customer.preferred_language in SUPPORTED_LANGUAGES:
                 return customer.preferred_language