fix(render+studio): dev mock worker (unstick the queue) + lock predefined layers

Render — "stuck in Queued" fix:
- Jobs were created Queued and only a Windows AE node could claim them, so in the
  dev stack (no node) they queued forever.
- New devworker package: in-process mock worker drives Queued jobs through the steps
  with progress + live preview frames → Done. Enabled via RENDER_DEV_WORKER (default
  true in compose; set false in prod where real nodes claim jobs).
- db: DevClaimNextQueued (atomic oldest-queued → Preparing) + UpdateJobStepProgress
- Verified live: a stuck job advanced Preparing→Done in ~10s with frontend polling.

Studio — predefined template structure:
- Projects are always copied from a template; structure is fixed. Users customise
  existing layers, they don't add new ones.
- New studio-config flag ALLOW_ADD_LAYERS (false): StudioToolbar (add text/image/
  video/shape) returns null; SceneEditSidebar "add text layer" button hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/studio/StudioToolbar.tsx

@@ -21,6 +21,7 @@ import {
 import type { ShapeKind } from "@/lib/studio-layer-props";
 import type { AddLayerInput } from "@/lib/studio-types";
 import { useStudioStore } from "@/lib/studio-store";
+import { ALLOW_ADD_LAYERS } from "@/lib/studio-config";
 
 const SHAPE_OPTIONS: {
   kind: ShapeKind;
@@ -94,6 +95,9 @@ export function StudioToolbar() {
   const videoInputRef = useRef<HTMLInputElement>(null);
   const [shapeOpen, setShapeOpen] = useState(false);
 
+  // Template projects have a predefined structure — adding new layers is disabled.
+  if (!ALLOW_ADD_LAYERS) return null;
+
   const handleAddText = () => {
     addLayer({
       type: "text",

src/components/studio/sidebar/SceneEditSidebarContent.tsx

@@ -6,6 +6,7 @@ import { useTranslations } from "next-intl";
 
 import { getTextProps, mergeLayerProps } from "@/lib/studio-layer-props";
 import { useStudioStore } from "@/lib/studio-store";
+import { ALLOW_ADD_LAYERS } from "@/lib/studio-config";
 
 export function SceneEditSidebarContent() {
   const t = useTranslations("auto.componentsStudioSidebarSceneEditSidebarContent");
@@ -137,17 +138,19 @@ export function SceneEditSidebarContent() {
         )}
       </div>
 
-      {/* Footer — add text button */}
-      <div className="shrink-0 border-t border-gray-200 p-3">
-        <button
-          type="button"
-          onClick={handleAddText}
-          className="flex w-full items-center justify-center gap-2 rounded-lg border border-dashed border-blue-300 bg-blue-50 px-3 py-2 text-xs font-medium text-blue-600 transition-colors hover:bg-blue-100 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-blue-500"
-        >
-          <Plus className="h-3.5 w-3.5" aria-hidden />
-          {t("addTextLayer")}
-        </button>
-      </div>
+      {/* Footer — add text button (hidden for predefined template projects) */}
+      {ALLOW_ADD_LAYERS && (
+        <div className="shrink-0 border-t border-gray-200 p-3">
+          <button
+            type="button"
+            onClick={handleAddText}
+            className="flex w-full items-center justify-center gap-2 rounded-lg border border-dashed border-blue-300 bg-blue-50 px-3 py-2 text-xs font-medium text-blue-600 transition-colors hover:bg-blue-100 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-blue-500"
+          >
+            <Plus className="h-3.5 w-3.5" aria-hidden />
+            {t("addTextLayer")}
+          </button>
+        </div>
+      )}
     </div>
   );
 }

src/lib/studio-config.ts

@@ -0,0 +1,17 @@
+/**
+ * Studio behaviour flags.
+ *
+ * FlatRender projects are always created FROM a template — the Studio service has
+ * no "blank project" concept and copies the template's scene graph into the editable
+ * project. The structure (scenes + layers) is therefore PREDEFINED: users customise
+ * existing layers (text, colours, images) but do not add or remove layers/scenes.
+ *
+ * Flip ALLOW_ADD_LAYERS to true only if a free-form (non-template) editor is ever
+ * introduced.
+ */
+
+/** Whether the user may add brand-new layers to the canvas. */
+export const ALLOW_ADD_LAYERS = false;
+
+/** Whether the user may add / delete / duplicate whole scenes. */
+export const ALLOW_EDIT_SCENES = false;