feat: cancel stale deployments when same sub deploys
When a new deployment is created for a subscription, any existing stale
deployments for that subscription are transitioned to a new :canceled state.
Also updates reconcile_deployments_on_connect to use :canceled instead of
:stale for explicitly superseded deployments.
sow-117
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>