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()