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/internal/db/db.go

@@ -519,6 +519,18 @@ func (s *Store) ClaimJob(ctx context.Context, nodeID uuid.UUID, region string) (
 	return s.getJobByIDInternal(ctx, jobID)
 }
 
+// UpdateJobPreview stores a base64-encoded preview frame for a running job.
+// Called by the node agent every N frames to power the live preview UI.
+func (s *Store) UpdateJobPreview(ctx context.Context, jobID uuid.UUID, imageB64 string) error {
+	_, err := s.pool.Exec(ctx, `
+		UPDATE render.render_jobs
+		SET image_preview_b64 = $1, updated_at = NOW()
+		WHERE id = $2
+		  AND step NOT IN ('Done'::render_step, 'Failed'::render_step, 'Cancelled'::render_step)`,
+		imageB64, jobID)
+	return err
+}
+
 func (s *Store) CancelJob(ctx context.Context, id, userID uuid.UUID) (bool, error) {
 	tag, err := s.pool.Exec(ctx, `
 		UPDATE render.render_jobs

services/render/internal/handlers/internal.go

@@ -198,6 +198,29 @@ func (h *InternalHandler) ReplicaReady(c *gin.Context) {
 	c.Status(http.StatusNoContent)
 }
 
+// POST /v1/internal/render/jobs/:job_id/preview
+// Node agent pushes a base64-encoded frame image so the frontend can show
+// a live preview while the job is rendering.
+func (h *InternalHandler) Preview(c *gin.Context) {
+	jobID, err := uuid.Parse(c.Param("job_id"))
+	if err != nil {
+		c.JSON(http.StatusBadRequest, models.APIError{Code: "bad_request", Message: "invalid job_id"})
+		return
+	}
+	var req struct {
+		ImageB64 string `json:"image_b64" binding:"required"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, models.APIError{Code: "bad_request", Message: err.Error()})
+		return
+	}
+	if err := h.store.UpdateJobPreview(c.Request.Context(), jobID, req.ImageB64); err != nil {
+		c.JSON(http.StatusInternalServerError, models.APIError{Code: "internal_error", Message: err.Error()})
+		return
+	}
+	c.Status(http.StatusNoContent)
+}
+
 // POST /v1/internal/render/jobs/claim
 // Node agent calls this to atomically claim the next queued job.
 // Returns 204 when there is nothing queued (agent should back off and retry).