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/handlers/app.go

@@ -314,7 +314,7 @@ func (h *Handlers) HandleAppDeploy() http.HandlerFunc {
 		deployCtx := context.WithoutCancel(request.Context())
 
 		go func(ctx context.Context, appToDeploy *models.App) {
-			deployErr := h.deploy.Deploy(ctx, appToDeploy, nil)
+			deployErr := h.deploy.Deploy(ctx, appToDeploy, nil, false)
 			if deployErr != nil {
 				h.log.Error(
 					"deployment failed",