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/cmd/server/main.go

@@ -82,6 +82,7 @@ func main() {
 	fontH    := handlers.NewFontHandler(store)
 	bundleH  := handlers.NewTemplateBundleHandler(mc, minioTemplatesBucket)
 	scanH    := handlers.NewScanHandler(store, mc, minioTemplatesBucket)
+	snapJobH := handlers.NewSnapshotJobHandler(store, mc, minioTemplatesBucket)
 	internalH := handlers.NewInternalHandler(store, notifyClient, mc, minioTemplatesBucket, minioBucket)
 
 	// ── Dev mock worker (no AE node needed) ────────────────────────────────────
@@ -90,6 +91,11 @@ func main() {
 	if devWorker {
 		go devworker.New(store).Run(context.Background())
 	}
+	// Snapshot-only dev mock: fulfils scene-snapshot jobs with a generated
+	// placeholder (no AE), gated separately so it never hijacks real render jobs.
+	if getEnv("RENDER_DEV_SNAPSHOTS", "false") == "true" {
+		go devworker.New(store).RunSnapshots(context.Background())
+	}
 
 	// ── Router ────────────────────────────────────────────────────────────────
 	r := gin.Default()
@@ -174,6 +180,10 @@ func main() {
 	v1.POST("/template-bundles/:project_id", auth, admin, bundleH.Set)
 
 	// ── Template scans (admin: read scenes/colours/configs from the AEP) ───────
+	// ── Scene snapshots (admin: render one frame per scene from AE) ────────────
+	v1.POST("/scene-snapshots/:project_id", auth, admin, snapJobH.Enqueue)
+	v1.GET("/scene-snapshots/:project_id", auth, admin, snapJobH.List)
+
 	v1.POST("/template-scans/:project_id/quick", auth, admin, scanH.QuickScan) // headless Go quick-scan
 	v1.POST("/template-scans/:project_id/jobs", auth, admin, scanH.CreateJob)  // queue an AE full scan
 	v1.GET("/template-scan-jobs/:id", auth, admin, scanH.GetJob)
@@ -205,6 +215,11 @@ func main() {
 		internal.POST("/render/jobs/:job_id/crash", internalH.Crash)
 		internal.POST("/render/jobs/:job_id/replica-ready", internalH.ReplicaReady)
 
+		// AE scene snapshots (node claims, renders one frame, posts the image URL)
+		internal.POST("/snapshot/claim", snapJobH.Claim)
+		internal.POST("/snapshot/:id/result", snapJobH.Result)
+		internal.POST("/snapshot/:id/fail", snapJobH.Fail)
+
 		// AE scan jobs (node claims, runs scan.jsx, posts the ScanResult back)
 		internal.POST("/scan/claim", scanH.Claim)
 		internal.GET("/scan/:id/status", scanH.Status) // node watchdog (cancel detection)