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/internal/client/client.go

@@ -188,6 +188,22 @@ func (c *Client) ClaimJob(ctx context.Context, nodeID, region string) (*ClaimedJ
 	return &job, nil
 }
 
+// UpdatePreview sends a base64-encoded preview frame to the orchestrator.
+// Errors are non-fatal — the UI simply won't update the preview image.
+func (c *Client) UpdatePreview(ctx context.Context, jobID, imageB64 string) error {
+	resp, err := c.do(ctx, http.MethodPost,
+		fmt.Sprintf("/v1/internal/render/jobs/%s/preview", jobID),
+		map[string]string{"image_b64": imageB64})
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode >= 300 {
+		return fmt.Errorf("preview: HTTP %d", resp.StatusCode)
+	}
+	return nil
+}
+
 // Complete marks a render job as Done.
 func (c *Client) Complete(ctx context.Context, jobID string, exportID *string) error {
 	resp, err := c.do(ctx, http.MethodPost,