refactor: update admin/vendor API imports to use module locations

Update all admin and vendor API routes to import schemas from
canonical module locations instead of legacy re-export files:
- messages: use app.modules.messaging.models/schemas
- customers: use app.modules.customers.schemas
- orders: use app.modules.orders.schemas
- inventory: use app.modules.inventory.schemas
- invoices: use app.modules.orders.schemas

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

app/api/v1/admin/messages.py

@@ -27,11 +27,11 @@ from app.exceptions import (
 )
 from app.services.message_attachment_service import message_attachment_service
 from app.services.messaging_service import messaging_service
-from models.database.message import ConversationType, ParticipantType
-from models.database.user import User
-from models.schema.message import (
+from app.modules.messaging.models import ConversationType, ParticipantType
+from app.modules.messaging.schemas import (
     AdminConversationListResponse,
     AdminConversationSummary,
+    AttachmentResponse,
     CloseConversationResponse,
     ConversationCreate,
     ConversationDetailResponse,
@@ -46,7 +46,7 @@ from models.schema.message import (
     ReopenConversationResponse,
     UnreadCountResponse,
 )
-from models.schema.message import AttachmentResponse
+from models.database.user import User
 
 router = APIRouter(prefix="/messages")
 logger = logging.getLogger(__name__)