feat(fase4): add GlobalVariable database model

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

services/api-gateway/app/models/__init__.py

@@ -3,6 +3,7 @@ from app.models.whatsapp import WhatsAppAccount, Contact, Conversation, Message
 from app.models.flow import Flow, FlowSession, TriggerType
 from app.models.queue import Queue, QueueAgent, AssignmentMethod
 from app.models.quick_reply import QuickReply
+from app.models.global_variable import GlobalVariable
 
 __all__ = [
     "User",
@@ -17,4 +18,5 @@ __all__ = [
     "QueueAgent",
     "AssignmentMethod",
     "QuickReply",
+    "GlobalVariable",
 ]