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/integration/api/v1/admin/test_messages.py

@@ -7,7 +7,7 @@ Tests the /api/v1/admin/messages/* endpoints.
 
 import pytest
 
-from models.database.message import ConversationType, ParticipantType
+from app.modules.messaging.models import ConversationType, ParticipantType
 
 
 @pytest.mark.integration