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>

docker-compose.v2.yml

@@ -196,6 +196,9 @@ services:
       IDENTITY_URL: "http://identity-svc:8080"
       SERVICE_TOKEN: "${SERVICE_TOKEN:-internal-service-secret}"
       PORT: "8080"
+      # Dev: process Queued jobs in-process (progress + preview → Done) without a
+      # Windows AE node. Set "false" in production where real render nodes claim jobs.
+      RENDER_DEV_WORKER: "${RENDER_DEV_WORKER:-true}"
     depends_on:
       postgres:
         condition: service_healthy