refactor: remove triggered_by field from webhook triggers and use automatic sync

- Remove triggered_by field from WorkflowWebhookTrigger model
- Replace manual webhook creation/deletion APIs with automatic sync via WebhookService
- Keep only GET API for retrieving webhook information
- Use same webhook ID for both debug and production environments (differentiated by endpoint)
- Add sync_webhook_relationships to automatically manage webhook lifecycle
- Update tests to remove triggered_by references
- Clean up unused imports and fix type checking issues

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

api/fields/workflow_trigger_fields.py

@@ -21,6 +21,5 @@ webhook_trigger_fields = {
     "webhook_url": fields.String,
     "webhook_debug_url": fields.String,
     "node_id": fields.String,
-    "triggered_by": fields.String,
     "created_at": fields.DateTime(dt_format="iso8601"),
 }