refactor: update test imports to use module locations

Update all test files to import from canonical module locations:
- Integration tests: orders, inventory, messages, invoices
- Unit tests: services and models
- Fixtures: customer, vendor, message fixtures

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/models/database/test_product.py

@@ -306,7 +306,7 @@ class TestProductInventoryProperties:
 
     def test_physical_product_with_inventory(self, db, test_vendor):
         """Test physical product calculates inventory from entries."""
-        from models.database.inventory import Inventory
+        from app.modules.inventory.models import Inventory
 
         product = Product(
             vendor_id=test_vendor.id,
@@ -364,7 +364,7 @@ class TestProductInventoryProperties:
 
     def test_digital_product_ignores_inventory_entries(self, db, test_vendor):
         """Test digital product returns unlimited even with inventory entries."""
-        from models.database.inventory import Inventory
+        from app.modules.inventory.models import Inventory
 
         product = Product(
             vendor_id=test_vendor.id,