When a webhook-triggered deploy starts for an app that already has a deploy
in progress, the existing deploy is now cancelled via context cancellation
before the new deploy begins. This prevents silently lost webhook deploys.
Changes:
- Add per-app active deploy tracking with cancel func and done channel
- Deploy() accepts cancelExisting param: true for webhook, false for manual
- Cancelled deployments are marked with new 'cancelled' status
- Add ErrDeployCancelled sentinel error
- Add DeploymentStatusCancelled model constant
- Add comprehensive tests for cancellation mechanics
