Clip transcoding: ffmpeg in the backend image, panel recipe in the node docs

docs/code/nodes.md

@@ -110,6 +110,49 @@ ring and streams bytes to connected screens. New frames eventually replace
 old ones. A volatile artifact returned as a run result is copied to persistent
 storage; emitted frames alone are not recorded as run artifacts.
 
+### Making clips playable on an LVGL panel
+
+A wall panel that renders dashboards natively, such as an ESP32-P4 running
+openpanel, decodes only H.264 Constrained Baseline (in software, about 29 fps
+at 640×360) and MJPEG. It refuses clips straight from a phone or camera, which
+are usually Main or High profile at 1080p or more. Re-encode those with a node
+that takes a `video` input port `clip` and has a `video` output port
+`panel_clip`:
+
+```python
+import os
+import subprocess
+import tempfile
+
+import fluksio
+
+# H.264 Constrained Baseline, at most 640x360 and 15 fps, mono AAC.
+PANEL = [
+    "-c:v", "libx264", "-profile:v", "baseline", "-level:v", "3.1",
+    "-pix_fmt", "yuv420p", "-fpsmax", "15",
+    "-vf", "scale=w=640:h=360:force_original_aspect_ratio=decrease:force_divisible_by=2",
+    "-preset", "veryfast", "-crf", "23", "-g", "30",
+    "-c:a", "aac", "-ac", "1", "-b:a", "64k", "-movflags", "+faststart",
+]
+
+
+def process(clip):
+    source = fluksio.load_artifact(clip)
+    with tempfile.TemporaryDirectory() as tmp:
+        target = os.path.join(tmp, "panel.mp4")
+        subprocess.run(
+            ["ffmpeg", "-nostdin", "-loglevel", "error", "-i", source, *PANEL, target],
+            check=True,
+            timeout=600,
+        )
+        name = os.path.splitext(clip.get("name") or "clip")[0] + "-panel.mp4"
+        return {"panel_clip": fluksio.save_artifact(target, name, "video/mp4")}
+```
+
+Use it only for clips headed for such a panel. Browsers play the originals,
+and the result plays there too, just smaller. The Docker image ships `ffmpeg`;
+a [remote worker](workers.md) needs it on its `PATH`.
+
 ## Printing
 
 `print` and `fluksio.logger.info(...)` are captured in the node logs. The