feat: live render preview — node agent pushes PNG frames, frontend displays them in real time

render-svc:
- db.UpdateJobPreview(): writes base64 PNG to render_jobs.image_preview_b64
  (only on active jobs; Done/Failed/Cancelled rows ignored)
- POST /v1/internal/render/jobs/:job_id/preview — node agent endpoint
- Route registered under /v1/internal (nodeAuth)

node-agent:
- runner.PreviewFn callback type alongside ProgressFn
- runner.preview.go: GeneratePreviewB64(percent, quality, resolution)
  — pure stdlib (image/png + encoding/base64), no external deps
  — 320×180 dark frame with animated progress bar + colored indicator dots
- mock render: pushes a preview frame at every step (5→95%)
- real AE render: pushes a preview frame every 30s
- client.UpdatePreview(): POST /v1/internal/render/jobs/:job_id/preview
- main.go: onPreview callback wires client.UpdatePreview() into runner.Run()

frontend:
- render-jobs.ts: RenderJobRow.preview_b64 field; read from progress endpoint
- status/route.ts: previewB64 included in JSON response
- RenderModal: aspect-ratio preview pane during polling — shows spinner until
  first frame arrives, then live-updates with each poll (every 3s);
  step label overlaid as badge bottom-right

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/render/cmd/server/main.go

@@ -137,6 +137,7 @@ func main() {
 		internal.POST("/nodes/:node_id/online", internalH.Online)
 		internal.POST("/nodes/:node_id/cache-update", internalH.CacheUpdate)
 		internal.POST("/render/jobs/claim", internalH.Claim)
+		internal.POST("/render/jobs/:job_id/preview", internalH.Preview)
 		internal.POST("/render/jobs/:job_id/frames", internalH.FrameProgress)
 		internal.POST("/render/jobs/:job_id/complete", internalH.Complete)
 		internal.POST("/render/jobs/:job_id/fail", internalH.Fail)