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>

backend/db/migrations/30_render_snapshot_jobs.sql

@@ -0,0 +1,28 @@
+-- =====================================================================
+-- RENDER SCHEMA — scene snapshot jobs
+-- Async "render one frame per scene from After Effects" jobs. A node claims a
+-- queued snapshot, runs aerender for the scene's comp at a single frame, uploads
+-- the still to object storage, and posts back the image URL. render-svc then
+-- writes it onto content.scenes.snapshot_url (same DB, cross-schema) so the
+-- studio scene bar + admin show a real thumbnail.
+-- =====================================================================
+
+SET search_path TO render, public;
+
+CREATE TABLE IF NOT EXISTS snapshot_jobs (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    project_id  UUID NOT NULL,               -- content project (template variant)
+    scene_id    UUID NOT NULL,               -- content scene the snapshot belongs to
+    scene_key   TEXT NOT NULL,
+    comp_name   TEXT NOT NULL DEFAULT '',     -- AE comp to render (scene key / render comp)
+    frame       INT  NOT NULL DEFAULT 0,      -- frame to capture
+    status      TEXT NOT NULL DEFAULT 'queued', -- queued | running | done | error
+    node_id     UUID,
+    image_url   TEXT,
+    error       TEXT,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_snapshot_jobs_status  ON snapshot_jobs(status, created_at);
+CREATE INDEX IF NOT EXISTS idx_snapshot_jobs_project ON snapshot_jobs(project_id, created_at DESC);