feat: implement cascading delete for gateway and associated installed skills

backend/app/api/gateways.py

@@ -14,6 +14,7 @@ from app.db import crud
 from app.db.pagination import paginate
 from app.db.session import get_session
 from app.models.agents import Agent
+from app.models.gateway_installed_skills import GatewayInstalledSkill
 from app.models.gateways import Gateway
 from app.schemas.common import OkResponse
 from app.schemas.gateways import (
@@ -175,6 +176,12 @@ async def delete_gateway(
         await service.clear_agent_foreign_keys(agent_id=agent.id)
         await session.delete(agent)
 
+    installed_skills = await GatewayInstalledSkill.objects.filter_by(
+        gateway_id=gateway.id,
+    ).all(session)
+    for installed_skill in installed_skills:
+        await session.delete(installed_skill)
+
     await session.delete(gateway)
     await session.commit()
     return OkResponse()