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>

src/lib/render-jobs.ts

@@ -31,6 +31,8 @@ export interface RenderJobRow {
   progress_message: string | null;
   output_url: string | null;
   error_message: string | null;
+  /** Base64-encoded PNG preview frame pushed by the node agent. Null until first frame arrives. */
+  preview_b64: string | null;
 }
 
 // ── Helpers ──────────────────────────────────────────────────────────────────
@@ -111,6 +113,7 @@ export async function getRenderJob(
     step?: V2RenderStep;
     progress?: number;
     message?: string;
+    preview_b64?: string | null;
   } | null;
   if (!progress) return null;
 
@@ -152,6 +155,7 @@ export async function getRenderJob(
     output_url: outputUrl,
     error_message:
       status === "failed" ? (progress.message ?? "Render failed") : null,
+    preview_b64: progress.preview_b64 ?? null,
   };
 }