feat: add stage management UI, fix null stages

- Add Stage button with inline form (name, tag pattern, max instances, auto deploy)
- Delete stage button per stage header
- Add createStage/updateStage/deleteStage to API client
- Fix null stages crash on env page
- Fix null slices globally in respondJSON via reflection

web/src/lib/api.ts

@@ -9,6 +9,7 @@ import type {
 	Registry,
 	RegistryImage,
 	Settings,
+	Stage,
 	StageEnv,
 	Volume
 } from './types';
@@ -115,6 +116,20 @@ export function deleteProject(id: string): Promise<{ deleted: string }> {
 	return del<{ deleted: string }>(`/api/projects/${id}`);
 }
 
+// ── Stages ─────────────────────────────────────────────────────────
+
+export function createStage(projectId: string, data: Partial<Stage>): Promise<Stage> {
+	return post<Stage>(`/api/projects/${projectId}/stages`, data);
+}
+
+export function updateStage(projectId: string, stageId: string, data: Partial<Stage>): Promise<Stage> {
+	return put<Stage>(`/api/projects/${projectId}/stages/${stageId}`, data);
+}
+
+export function deleteStage(projectId: string, stageId: string): Promise<void> {
+	return del<void>(`/api/projects/${projectId}/stages/${stageId}`);
+}
+
 // ── Instances ───────────────────────────────────────────────────────
 
 export function listInstances(projectId: string, stageId: string): Promise<Instance[]> {