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]>

backend/main.py

@@ -21,6 +21,7 @@ from routes.settings_route import router as settings_router
 from config import settings
 from services.load_balancer import lb
 from services.node_store import load_nodes
+from services.settings_store import load_settings
 
 
 async def _origin_paths() -> dict[str, dict]:
@@ -150,6 +151,9 @@ async def monitor_origin():
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     load_nodes()
+    # Must run before anything reads settings: it applies the administrator's saved
+    # overrides (brand, transcode ladder) on top of the config defaults.
+    load_settings()
     await init_db()
     os.makedirs(settings.hls_runtime_dir, exist_ok=True)
     # Bring the media origin up before the transcoders: they immediately open RTMP