fix: cancel in-progress deploy on new webhook trigger

When a webhook-triggered deploy starts for an app that already has a deploy
in progress, the new deploy now cancels the existing one via context
cancellation, waits for the lock to be released, and then starts the new
deploy.

Changes:
- Add per-app context cancellation (appCancels sync.Map) to deploy.Service
- Deploy() creates a cancellable context and registers it for the app
- Add CancelAppDeploy() method to cancel an in-progress deploy
- Add ErrDeployCancelled sentinel error for cancelled deploys
- Handle context cancellation in build and deploy phases, marking
  deployments as failed with a clear cancellation message
- Webhook triggerDeployment() now cancels in-progress deploys and retries
  until the lock is released (up to 30 attempts with 2s delay)

fixes #38

internal/service/webhook/webhook.go

@@ -5,8 +5,10 @@ import (
 	"context"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
+	"time"
 
 	"go.uber.org/fx"
 
@@ -129,6 +131,12 @@ func (svc *Service) HandleWebhook(
 	return nil
 }
 
+// cancelRetryDelay is the time to wait after cancelling a deploy before retrying.
+const cancelRetryDelay = 2 * time.Second
+
+// cancelRetryAttempts is the maximum number of times to retry after cancelling.
+const cancelRetryAttempts = 30
+
 func (svc *Service) triggerDeployment(
 	ctx context.Context,
 	app *models.App,
@@ -144,6 +152,25 @@ func (svc *Service) triggerDeployment(
 		deployCtx := context.WithoutCancel(ctx)
 
 		deployErr := svc.deploy.Deploy(deployCtx, app, &eventID)
+		if deployErr != nil && errors.Is(deployErr, deploy.ErrDeploymentInProgress) {
+			// Cancel the in-progress deployment and retry
+			svc.log.Info("cancelling in-progress deployment for new webhook trigger", "app", appName)
+			svc.deploy.CancelAppDeploy(app.ID)
+
+			// Retry until the lock is released by the cancelled deploy
+			for attempt := range cancelRetryAttempts {
+				time.Sleep(cancelRetryDelay)
+
+				svc.log.Info("retrying deployment after cancel",
+					"app", appName, "attempt", attempt+1)
+
+				deployErr = svc.deploy.Deploy(deployCtx, app, &eventID)
+				if deployErr == nil || !errors.Is(deployErr, deploy.ErrDeploymentInProgress) {
+					break
+				}
+			}
+		}
+
 		if deployErr != nil {
 			svc.log.Error("deployment failed", "error", deployErr, "app", appName)
 		}