feat: add user-facing deployment cancel endpoint

Add POST /apps/{id}/deployments/cancel endpoint that allows users to
cancel in-progress deployments via the web UI.

Changes:
- Add CancelDeploy() and HasActiveDeploy() public methods to deploy service
- Add HandleCancelDeploy handler
- Wire route in routes.go
- Add cancel button to app detail template (shown during active deployments)
- Add handler tests for cancel endpoint

fixes #66

internal/handlers/app.go

@@ -354,6 +354,32 @@ func (h *Handlers) HandleAppDeploy() http.HandlerFunc {
 	}
 }
 
+// HandleCancelDeploy cancels an in-progress deployment for an app.
+func (h *Handlers) HandleCancelDeploy() http.HandlerFunc {
+	return func(writer http.ResponseWriter, request *http.Request) {
+		appID := chi.URLParam(request, "id")
+
+		application, findErr := models.FindApp(request.Context(), h.db, appID)
+		if findErr != nil || application == nil {
+			http.NotFound(writer, request)
+
+			return
+		}
+
+		cancelled := h.deploy.CancelDeploy(application.ID)
+		if cancelled {
+			h.log.Info("deployment cancelled by user", "app", application.Name)
+		}
+
+		http.Redirect(
+			writer,
+			request,
+			"/apps/"+application.ID,
+			http.StatusSeeOther,
+		)
+	}
+}
+
 // HandleAppDeployments returns the deployments history handler.
 func (h *Handlers) HandleAppDeployments() http.HandlerFunc {
 	tmpl := templates.GetParsed()

internal/handlers/handlers_test.go

@@ -684,6 +684,47 @@ func TestDeletePortOwnershipVerification(t *testing.T) {
 	assert.NotNil(t, found, "port should still exist after IDOR attempt")
 }
 
+func TestHandleCancelDeployRedirects(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+
+	createdApp := createTestApp(t, testCtx, "cancel-deploy-app")
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/apps/"+createdApp.ID+"/deployments/cancel",
+		nil,
+	)
+	request = addChiURLParams(request, map[string]string{"id": createdApp.ID})
+	recorder := httptest.NewRecorder()
+
+	handler := testCtx.handlers.HandleCancelDeploy()
+	handler.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusSeeOther, recorder.Code)
+	assert.Equal(t, "/apps/"+createdApp.ID, recorder.Header().Get("Location"))
+}
+
+func TestHandleCancelDeployReturns404ForUnknownApp(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/apps/nonexistent/deployments/cancel",
+		nil,
+	)
+	request = addChiURLParams(request, map[string]string{"id": "nonexistent"})
+	recorder := httptest.NewRecorder()
+
+	handler := testCtx.handlers.HandleCancelDeploy()
+	handler.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusNotFound, recorder.Code)
+}
+
 func TestHandleWebhookReturns404ForUnknownSecret(t *testing.T) {
 	t.Parallel()
 

internal/server/routes.go

@@ -68,6 +68,7 @@ func (s *Server) SetupRoutes() {
 		r.Post("/apps/{id}", s.handlers.HandleAppUpdate())
 		r.Post("/apps/{id}/delete", s.handlers.HandleAppDelete())
 		r.Post("/apps/{id}/deploy", s.handlers.HandleAppDeploy())
+		r.Post("/apps/{id}/deployments/cancel", s.handlers.HandleCancelDeploy())
 		r.Get("/apps/{id}/deployments", s.handlers.HandleAppDeployments())
 		r.Get("/apps/{id}/deployments/{deploymentID}/logs", s.handlers.HandleDeploymentLogsAPI())
 		r.Get("/apps/{id}/deployments/{deploymentID}/download", s.handlers.HandleDeploymentLogDownload())

internal/service/deploy/deploy.go

@@ -283,6 +283,26 @@ func (svc *Service) GetLogFilePath(app *models.App, deployment *models.Deploymen
 	return filepath.Join(svc.config.DataDir, "logs", hostname, app.Name, filename)
 }
 
+// HasActiveDeploy returns true if there is an active deployment for the given app.
+func (svc *Service) HasActiveDeploy(appID string) bool {
+	_, ok := svc.activeDeploys.Load(appID)
+
+	return ok
+}
+
+// CancelDeploy cancels any in-progress deployment for the given app
+// and waits for it to finish before returning. Returns true if a deployment
+// was cancelled, false if there was nothing to cancel.
+func (svc *Service) CancelDeploy(appID string) bool {
+	if !svc.HasActiveDeploy(appID) {
+		return false
+	}
+
+	svc.cancelActiveDeploy(appID)
+
+	return true
+}
+
 // Deploy deploys an app. If cancelExisting is true (e.g. webhook-triggered),
 // any in-progress deploy for the same app will be cancelled before starting.
 // If cancelExisting is false and a deploy is in progress, ErrDeploymentInProgress is returned.

templates/app_detail.html

@@ -40,6 +40,10 @@
                     <span x-text="deploying ? 'Deploying...' : 'Deploy Now'"></span>
                 </button>
             </form>
+            <form method="POST" action="/apps/{{.App.ID}}/deployments/cancel" class="inline" x-show="deploying" x-cloak x-data="confirmAction('Cancel the current deployment?')" @submit="confirm($event)">
+                {{ .CSRFField }}
+                <button type="submit" class="btn-danger">Cancel Deploy</button>
+            </form>
         </div>
     </div>