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/node-agent/cmd/agent/main.go

@@ -246,12 +246,20 @@ func (a *Agent) runJob(ctx context.Context, job *client.ClaimedJob) {
 	}
 
 	onProgress := func(ctx context.Context, pct int, msg string) error {
-		// Lightweight progress update — errors here are non-fatal
 		log.Printf("[job %s] %d%% %s", job.JobID, pct, msg)
 		return nil
 	}
 
-	outputPath, err := runner.Run(ctx, a.cfg.AEPath, a.cfg.WorkDir, rJob, onProgress)
+	onPreview := func(ctx context.Context, imageB64 string) error {
+		pvCtx, cancel := context.WithTimeout(ctx, 8*time.Second)
+		defer cancel()
+		if err := a.orch.UpdatePreview(pvCtx, job.JobID, imageB64); err != nil {
+			log.Printf("[job %s] preview push error: %v", job.JobID, err)
+		}
+		return nil
+	}
+
+	outputPath, err := runner.Run(ctx, a.cfg.AEPath, a.cfg.WorkDir, rJob, onProgress, onPreview)
 	if err != nil {
 		if ctx.Err() != nil {
 			log.Printf("[job %s] render cancelled", job.JobID)