Fix media stop, upload cleanup and review findings

- Stop is a no-op when the collected player unit is already gone
  (raw systemctl stop exits 5 on the Frame; verified 2026-09-29).
- Surface systemd-run stderr when the player can't start.
- Keep the copy error if the cleanup ssh also fails; reject upload
  names that the play path can never accept.
- Allow 60 s for play (ffprobe 30 s + systemd-run 15 s remote).
- Docs: four-hour cap is unconditional; no delete action yet; fix a
  garbled timing sentence.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/vr-video.md

@@ -53,9 +53,9 @@ All test media were generated by us. No paid content or DRM was involved.
   test in 0.128 s (decode only); converting all frames to RGBA took 0.242 s.
 - Our ffmpeg → Python → OpenVR path submitted all 150 frames and exited 0.
   The 1280×720 prototype took 4.775 s; the full 1920×1080 player took
-  4.618 s. These are wall times, not in-headset frame-rate measurements;
-  These initial probes exposed an early finish. The final player explicitly
-  paces 30 fps output: all 150 frames then completed in **5.025 s**.
+  4.618 s. These are wall times, not in-headset frame-rate measurements.
+  Finishing a 5 s clip early showed those probes weren't paced, so the final
+  player paces output at 30 fps: all 150 frames then completed in **5.025 s**.
 - The H.265 hardware decoder also completed the generated 1080p clip
   (60 frames, exit 0); this is a short compatibility check, not a 4K/8K benchmark.
 - Our actual player, launched through Frame Control's media API, displayed
@@ -101,8 +101,9 @@ The ffmpeg path above completed instead.
   version rejects files above 20,000 records. It is a stereo preview, not
   an immersive walk-through.
 - A single player can run at a time. It stops at EOF, on **Stop**, or after
-  four hours if the computer disconnects. Still images remain until Stop
-  or that timeout. The log is
+  four hours in any case, so longer movies are cut off. Still images remain
+  until Stop or that timeout. There is no delete action yet: remove old
+  uploads from `~/Videos/FrameControl/` over SSH. The log is
   `~/.local/share/frame-control/media/player.log` on the Frame.
 - **Panel/stream theatre remains outside this media slice:** SteamVR's
   `vrcmd --dock-overlay` accepts `theater`, but docking an existing panel