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/tenancy/services/merchant_service.py

@@ -125,6 +125,21 @@ class MerchantService:
 
         return merchant
 
+    def get_merchant_by_id_optional(
+        self, db: Session, merchant_id: int
+    ) -> Merchant | None:
+        """
+        Get merchant by ID, returns None if not found.
+
+        Args:
+            db: Database session
+            merchant_id: Merchant ID
+
+        Returns:
+            Merchant object or None
+        """
+        return db.query(Merchant).filter(Merchant.id == merchant_id).first()
+
     def get_merchants(
         self,
         db: Session,