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>

services/render/internal/devworker/devworker.go

@@ -102,6 +102,62 @@ func (w *Worker) simulate(ctx context.Context, jobID uuid.UUID) {
 	log.Printf("[devworker] job %s done", jobID)
 }
 
+// ── Snapshot mock ─────────────────────────────────────────────────────────────
+// devSnapshotNode is the synthetic node id the mock records on claimed snapshots.
+var devSnapshotNode = uuid.MustParse("00000000-0000-0000-0000-0000000000aa")
+
+// RunSnapshots fulfils queued scene-snapshot jobs with a generated placeholder
+// image (no AE) so the snapshot flow is exercisable in development. Gated by its
+// own flag so it never touches real render jobs. Production uses real nodes.
+func (w *Worker) RunSnapshots(ctx context.Context) {
+	log.Printf("[devworker] snapshot mock started (poll %s) — NOT for production", w.interval)
+	ticker := time.NewTicker(w.interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			w.snapTick(ctx)
+		}
+	}
+}
+
+func (w *Worker) snapTick(ctx context.Context) {
+	claim, err := w.store.ClaimSnapshotJob(ctx, devSnapshotNode)
+	if err != nil {
+		log.Printf("[devworker] snapshot claim error: %v", err)
+		return
+	}
+	if claim == nil {
+		return // queue empty
+	}
+	if err := w.store.SetSnapshotResult(ctx, claim.ID, snapshotPlaceholder(claim.SceneKey)); err != nil {
+		log.Printf("[devworker] snapshot %s result failed: %v", claim.ID, err)
+		_ = w.store.SetSnapshotError(ctx, claim.ID, err.Error())
+		return
+	}
+	log.Printf("[devworker] snapshot %s (scene %s) done", claim.ID, claim.SceneKey)
+}
+
+// snapshotPlaceholder builds a 480×270 PNG card tinted by the scene key with a
+// little "play" block, returned as a data: URL so the dev path needs no storage.
+func snapshotPlaceholder(sceneKey string) string {
+	const w, h = 480, 270
+	var sum uint32
+	for _, r := range sceneKey {
+		sum = sum*31 + uint32(r)
+	}
+	base := color.RGBA{uint8(40 + sum%120), uint8(40 + (sum/120)%120), uint8(80 + (sum/7)%150), 255}
+	img := image.NewRGBA(image.Rect(0, 0, w, h))
+	draw.Draw(img, img.Bounds(), &image.Uniform{base}, image.Point{}, draw.Src)
+	draw.Draw(img, image.Rect(0, h/2-30, w, h/2+30), &image.Uniform{color.RGBA{0, 0, 0, 60}}, image.Point{}, draw.Over)
+	draw.Draw(img, image.Rect(w/2-18, h/2-18, w/2+18, h/2+18), &image.Uniform{color.RGBA{255, 255, 255, 230}}, image.Point{}, draw.Over)
+	var buf bytes.Buffer
+	_ = png.Encode(&buf, img)
+	return "data:image/png;base64," + base64.StdEncoding.EncodeToString(buf.Bytes())
+}
+
 // previewB64 builds a 320×180 PNG with a progress bar — same idea as the node
 // agent's GeneratePreviewB64, kept local so render-svc has no node-agent dep.
 func previewB64(pct int) string {