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/services/settings_store.py

@@ -0,0 +1,55 @@
+"""Persist the settings an administrator edits in the UI.
+
+config.py holds the defaults, but anything changed at runtime has to survive a
+restart — otherwise editing the brand or the transcode ladder appears to work and
+is silently reverted the next time the backend starts. Writes are atomic (temp
+file + replace) so a crash mid-save cannot leave a half-written file that fails
+to parse on boot.
+"""
+
+import json
+from pathlib import Path
+
+from config import settings
+
+STORE_PATH = Path(__file__).resolve().parent.parent / "settings.json"
+
+# Settings the UI may change, with the attribute they map to on Settings.
+_MANAGED = (
+    "brand_name",
+    "brand_tagline",
+    "brand_logo_url",
+    "load_balance_strategy",
+    "multicast_enabled",
+    "transcode_profiles",
+    "qualities",
+)
+
+
+def load_settings() -> None:
+    """Apply saved overrides on top of the config defaults. Never raises."""
+    if not STORE_PATH.exists():
+        return
+    try:
+        saved = json.loads(STORE_PATH.read_text(encoding="utf-8"))
+    except (OSError, json.JSONDecodeError):
+        # A corrupt file must not stop the server from booting; the defaults from
+        # config.py are still usable.
+        return
+    if not isinstance(saved, dict):
+        return
+    for key, value in saved.items():
+        # Only known keys, so a stale or hand-edited file cannot inject arbitrary
+        # attributes onto the settings object.
+        if key in _MANAGED and value is not None:
+            setattr(settings, key, value)
+
+
+def save_settings() -> None:
+    payload = {key: getattr(settings, key) for key in _MANAGED}
+    temporary = STORE_PATH.with_suffix(".json.tmp")
+    temporary.write_text(
+        json.dumps(payload, ensure_ascii=False, indent=2),
+        encoding="utf-8",
+    )
+    temporary.replace(STORE_PATH)