feat(snapshots): AE scene-snapshot pipeline + admin trigger (Epic C, C1)

Per-scene preview thumbnails for templates. Admin clicks "ساخت پیش‌نمایش
صحنه‌ها" → one single-frame AE render per scene → content.scenes.snapshot_url
→ shown as a thumbnail in the admin scene list (and available to the studio).

- migration 30_render_snapshot_jobs.sql: render.snapshot_jobs (queued|running|
  done|error, per scene, image_url).
- render-svc: db/snapshotjobs.go (EnqueueSceneSnapshots, List, Claim, SetResult
  -> writes content.scenes.snapshot_url cross-schema, SetError); handlers/
  snapshotjobs.go (admin POST/GET /v1/scene-snapshots/:project_id + node-facing
  internal claim/result/fail); main.go routes; gateway route.
- devworker: RunSnapshots — fulfils snapshot jobs with a generated placeholder
  PNG (data: URL, scene-key-tinted) so the flow is verifiable without an AE node.
  Gated by RENDER_DEV_SNAPSHOTS (default off; never hijacks real render jobs).
- admin UI: ProjectScenes "generate snapshots" button (enqueue + poll + reload)
  and a thumbnail (snapshot_url || image) per scene row.

Verified e2e via the dev mock: enqueue -> jobs run -> content.scenes.snapshot_url
populated -> scenes API returns it -> admin renders the thumbnail.

Remaining (C2): node-agent real-AE runner — claim snapshot, aerender -s0 -e0 ->
ffmpeg still -> upload to a PERMANENT URL (mirror file-svc, not the time-limited
export presign) -> post result. Needs a live AE node to build + verify.

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

docker-compose.v2.yml

@@ -200,6 +200,9 @@ services:
       # 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: "${RENDER_DEV_WORKER:-true}"
+      # Dev: fulfil scene-snapshot jobs with a generated placeholder image (no AE).
+      # Keep "false" in production — real nodes render the actual AE frame.
+      RENDER_DEV_SNAPSHOTS: "${RENDER_DEV_SNAPSHOTS:-false}"
     depends_on:
       postgres:
         condition: service_healthy