feat: keyboard seek, playback speed, and persist edited settings

Player
- Arrow keys seek by 5s, Shift for a 30s jump on long recordings, J/L for +-30s,
  Space toggles playback, and up/down adjust volume. Dragging a scrubber across a
  multi-hour replay is unusable — one pixel is tens of seconds — so keys are the
  practical way to move around a long recording.
- The shortcuts ignore key events from inputs, so typing a chapter time in the
  replay editor no longer scrubs the video.
- Add a playback speed selector. It is a replay tool: speeding a live stream up
  would only push it further ahead of the encoder's buffer.

Replay editor
- Stop playback restarting on every edit. The player's source effect depended on
  the `lines` and `qualities` arrays, and callers such as the replay editor pass
  fresh literals on every render, so adding a chapter or typing a title was
  treated as a source change and reloaded the stream from the beginning. The
  effect now depends on the resolved source URL instead of the arrays it came
  from. Measured: adding three chapters leaves the media requests unchanged and
  the playhead advancing.

Settings persistence
- Settings edited in the UI were applied to the in-memory settings object and
  never written down, so the brand, transcode ladder and load-balancing strategy
  silently reverted to the config.py defaults on every restart. They are now
  saved to backend/settings.json (atomic temp-file replace) and re-applied at
  startup, with only known keys accepted so a hand-edited file cannot inject
  arbitrary attributes.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

.gitignore

@@ -22,6 +22,8 @@ media/
 *.key
 # written by the admin node manager, not part of the source
 backend/nodes.json
+# administrator-edited settings (brand, transcode ladder) saved at runtime
+backend/settings.json
 
 # MediaMTX binaries and archives are fetched, not committed
 mediamtx/mediamtx