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/runner/runner.go

@@ -1,5 +1,5 @@
 // Package runner executes After Effects render jobs and streams progress back
-// via the provided callback. When AE_PATH is empty, a mock render is used
+// via the provided callbacks. When AE_PATH is empty, a mock render is used
 // (useful for CI and dev environments without a licensed AE installation).
 package runner
 
@@ -16,6 +16,10 @@ import (
 // ProgressFn is called periodically during rendering with (percent 0-100, message).
 type ProgressFn func(ctx context.Context, percent int, message string) error
 
+// PreviewFn is called each time a new preview frame is ready.
+// The argument is a base64-encoded PNG. Errors are non-fatal.
+type PreviewFn func(ctx context.Context, imageB64 string) error
+
 // Job holds the parameters for a single render.
 type Job struct {
 	JobID          string
@@ -30,9 +34,9 @@ type Job struct {
 	AEPFilePath string
 }
 
-// Run executes the render job, calling onProgress as it advances.
+// Run executes the render job, calling onProgress and onPreview as it advances.
 // Returns the path to the output MP4 file on success.
-func Run(ctx context.Context, aePath, workDir string, job *Job, onProgress ProgressFn) (string, error) {
+func Run(ctx context.Context, aePath, workDir string, job *Job, onProgress ProgressFn, onPreview PreviewFn) (string, error) {
 	outputDir := filepath.Join(workDir, "renders", job.JobID)
 	if err := os.MkdirAll(outputDir, 0o755); err != nil {
 		return "", fmt.Errorf("create output dir: %w", err)
@@ -40,14 +44,14 @@ func Run(ctx context.Context, aePath, workDir string, job *Job, onProgress Progr
 	outputPath := filepath.Join(outputDir, "output.mp4")
 
 	if aePath == "" {
-		return mockRender(ctx, job, outputPath, onProgress)
+		return mockRender(ctx, job, outputPath, onProgress, onPreview)
 	}
-	return aeRender(ctx, aePath, job, outputPath, onProgress)
+	return aeRender(ctx, aePath, job, outputPath, onProgress, onPreview)
 }
 
 // ── Mock render (no AE installed) ────────────────────────────────────────────
 
-func mockRender(ctx context.Context, job *Job, outputPath string, onProgress ProgressFn) (string, error) {
+func mockRender(ctx context.Context, job *Job, outputPath string, onProgress ProgressFn, onPreview PreviewFn) (string, error) {
 	log.Printf("[mock] starting render for job %s (%s %s %dfps)", job.JobID, job.Quality, job.Resolution, job.FrameRate)
 
 	steps := []struct {
@@ -72,6 +76,13 @@ func mockRender(ctx context.Context, job *Job, outputPath string, onProgress Pro
 		if err := onProgress(ctx, s.pct, s.msg); err != nil {
 			log.Printf("[mock] progress callback error: %v", err)
 		}
+		// Generate and push a preview frame at each step
+		if onPreview != nil {
+			b64 := GeneratePreviewB64(s.pct, job.Quality, job.Resolution)
+			if err := onPreview(ctx, b64); err != nil {
+				log.Printf("[mock] preview callback error: %v", err)
+			}
+		}
 		log.Printf("[mock] %d%% — %s", s.pct, s.msg)
 	}
 
@@ -85,7 +96,7 @@ func mockRender(ctx context.Context, job *Job, outputPath string, onProgress Pro
 
 // ── Real AE render via aerender.exe ──────────────────────────────────────────
 
-func aeRender(ctx context.Context, aePath string, job *Job, outputPath string, onProgress ProgressFn) (string, error) {
+func aeRender(ctx context.Context, aePath string, job *Job, outputPath string, onProgress ProgressFn, onPreview PreviewFn) (string, error) {
 	if job.AEPFilePath == "" {
 		return "", fmt.Errorf("AEPFilePath is required for real AE render")
 	}
@@ -93,9 +104,6 @@ func aeRender(ctx context.Context, aePath string, job *Job, outputPath string, o
 	// aerender flags:
 	//   -project <path.aep>
 	//   -output  <output.mp4>
-	//   -RStemplate "Multi-Machine Settings"   (optional)
-	//   -OMtemplate "H.264 – Match Render Settings – 15 Mbps"
-	//   -s <start_frame> -e <end_frame>
 	args := []string{
 		"-project", job.AEPFilePath,
 		"-output", outputPath,
@@ -120,6 +128,11 @@ func aeRender(ctx context.Context, aePath string, job *Job, outputPath string, o
 	ticker := time.NewTicker(10 * time.Second)
 	defer ticker.Stop()
 
+	// Generate preview frames every 30 seconds during real AE render.
+	// In a full implementation this would screenshot the AE composition output.
+	previewTicker := time.NewTicker(30 * time.Second)
+	defer previewTicker.Stop()
+
 	for {
 		select {
 		case err := <-done:
@@ -133,6 +146,13 @@ func aeRender(ctx context.Context, aePath string, job *Job, outputPath string, o
 				pct += 5
 			}
 			_ = onProgress(ctx, pct, fmt.Sprintf("Rendering… %d%%", pct))
+		case <-previewTicker.C:
+			if onPreview != nil {
+				b64 := GeneratePreviewB64(pct, job.Quality, job.Resolution)
+				if err := onPreview(ctx, b64); err != nil {
+					log.Printf("[ae] preview push error: %v", err)
+				}
+			}
 		case <-ctx.Done():
 			_ = cmd.Process.Kill()
 			return "", ctx.Err()