feat(render): real progress %, ETA, and frequent preview during AE renders

The render page already displayed progress/ETA/preview — but the node agent never
fed real data: aeRender used fake +5%/10s increments, discarded aerender stdout,
and pushed a preview only every 30s. (Plus the deployed agent predated even the
progress-reporting wiring.)

node-agent (aeRender):
- Capture aerender stdout; parse "(N):" current frame + "N frames"/"to N" total.
- Real percentage when total is known (5–90%, headroom for transcode/upload),
  else a smooth time-asymptotic estimate that never sticks — message shows the
  live frame number either way.
- Push a preview frame ~every 8s (was 30s) so the box fills in quickly.

render-svc:
- GET /v1/renders/:id/progress now computes eta_seconds from started_at + progress
  (linear extrapolation) instead of returning null.

frontend:
- Thread eta_seconds → status route → render page; page prefers the server ETA and
  falls back to the client-observed rate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/render/internal/handlers/renders.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"time"
 	"log"
 	"net/http"
 	"strconv"
@@ -259,13 +260,29 @@ func (h *RenderHandler) Progress(c *gin.Context) {
 		c.JSON(http.StatusNotFound, models.APIError{Code: "not_found", Message: err.Error()})
 		return
 	}
+
+	// Estimate remaining seconds from elapsed time and progress (linear extrapolation).
+	// Only once the job is actually progressing (5–99%) and we know when it started.
+	var etaSeconds *int
+	if job.StartedAt != nil && job.RenderProgress >= 5 && job.RenderProgress < 100 {
+		elapsed := time.Since(*job.StartedAt).Seconds()
+		if elapsed > 0 {
+			remaining := elapsed * float64(100-job.RenderProgress) / float64(job.RenderProgress)
+			if remaining < 0 {
+				remaining = 0
+			}
+			eta := int(remaining)
+			etaSeconds = &eta
+		}
+	}
+
 	c.JSON(http.StatusOK, gin.H{
 		"job_id":         job.ID,
 		"step":           job.Step,
 		"progress":       job.RenderProgress,
 		"current_frame":  nil,
 		"total_frames":   nil,
-		"eta_seconds":    nil,
+		"eta_seconds":    etaSeconds,
 		"preview_b64":    job.ImagePreviewB64,
 		"active_nodes":   job.CurrentActiveNodes,
 		"message":        job.FailedMessage,