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/fixtures/message_fixtures.py

@@ -8,7 +8,7 @@ See tests/conftest.py for details on fixture best practices.
 
 import pytest
 
-from models.database.message import (
+from app.modules.messaging.models import (
     Conversation,
     ConversationParticipant,
     ConversationType,