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/marketplace/services/onboarding_service.py

@@ -31,7 +31,6 @@ from app.modules.marketplace.services.letzshop import (
     LetzshopOrderService,
 )
 from app.modules.tenancy.exceptions import StoreNotFoundException
-from app.modules.tenancy.models import Store
 
 logger = logging.getLogger(__name__)
 
@@ -52,6 +51,12 @@ class OnboardingService:
         """
         self.db = db
 
+    def _get_store(self, store_id: int):
+        """Get store by ID via store service."""
+        from app.modules.tenancy.services.store_service import store_service
+
+        return store_service.get_store_by_id_optional(self.db, store_id)
+
     # =========================================================================
     # Onboarding CRUD
     # =========================================================================
@@ -167,7 +172,7 @@ class OnboardingService:
 
     def get_merchant_profile_data(self, store_id: int) -> dict:
         """Get current merchant profile data for editing."""
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         if not store:
             return {}
 
@@ -206,7 +211,7 @@ class OnboardingService:
         Returns response with next step information.
         """
         # Check store exists BEFORE creating onboarding record (FK constraint)
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         if not store:
             raise StoreNotFoundException(store_id)
 
@@ -346,7 +351,7 @@ class OnboardingService:
         )
 
         # Update store with Letzshop identity
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         if store:
             store.letzshop_store_slug = shop_slug
             if letzshop_store_id:
@@ -374,7 +379,7 @@ class OnboardingService:
 
     def get_product_import_config(self, store_id: int) -> dict:
         """Get current product import configuration."""
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         if not store:
             return {}
 
@@ -422,7 +427,7 @@ class OnboardingService:
             raise OnboardingCsvUrlRequiredException()
 
         # Update store settings
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         if not store:
             raise StoreNotFoundException(store_id)
 
@@ -607,7 +612,7 @@ class OnboardingService:
         self.db.flush()
 
         # Get store code for redirect URL
-        store = self.db.query(Store).filter(Store.id == store_id).first()
+        store = self._get_store(store_id)
         store_code = store.store_code if store else ""
 
         logger.info(f"Completed onboarding for store {store_id}")