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>

tests/unit/services/test_stats_service.py

@@ -495,7 +495,10 @@ class TestStatsService:
         )
         db.commit()
 
-        count = self.service._get_unique_brands_count(db)
+        from app.modules.marketplace.services.marketplace_product_service import (
+            marketplace_product_service,
+        )
+        count = marketplace_product_service.get_distinct_brand_count(db)
 
         assert count >= 2  # At least BrandA and BrandB
         assert isinstance(count, int)
@@ -525,7 +528,10 @@ class TestStatsService:
         )
         db.commit()
 
-        count = self.service._get_unique_categories_count(db)
+        from app.modules.marketplace.services.marketplace_product_service import (
+            marketplace_product_service,
+        )
+        count = marketplace_product_service.get_distinct_category_count(db)
 
         assert count >= 2  # At least Electronics and Books
         assert isinstance(count, int)