fix: cancel in-progress deploy when webhook triggers new deploy (closes #38)

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

internal/models/deployment.go

@@ -19,6 +19,7 @@ const (
 	DeploymentStatusDeploying DeploymentStatus = "deploying"
 	DeploymentStatusSuccess   DeploymentStatus = "success"
 	DeploymentStatusFailed    DeploymentStatus = "failed"
+	DeploymentStatusCancelled DeploymentStatus = "cancelled"
 )
 
 // Display constants.