feat(playlists): expose scene playlists as Home Assistant entities

One device per scene playlist (model "Scene Playlist"), each with:
- a switch (start/stop; the server cycles one playlist at a time, so starting one stops any other)
- a "Current Scene" sensor that resolves the active preset name from scene_presets while this playlist is cycling
- an "Items" diagnostic sensor (configured scene count; no state_class)

The coordinator reads scene_playlists plus the flat playlist_state from the /api/v1/snapshot payload and gains start_playlist()/stop_playlist(); __init__ registers and prunes per-playlist devices and reloads on playlist-id changes; the event listener also refreshes on the playlist_state_changed WS event. Shared device/lookup/running-state plumbing lives in a new LedGrabPlaylistEntity base (entity.py) used by both the switch and the sensors. Adds en/ru translation keys.

Note: this also lands the in-progress coordinator migration from the per-request fan-out to the single /api/v1/snapshot poll that was already present in the working tree.

Requires the led-grab server /api/v1/snapshot to emit scene_playlists + playlist_state (companion server change, tracked separately).

custom_components/ledgrab/entity.py

@@ -0,0 +1,58 @@
+"""Shared base entity for scene-playlist platforms.
+
+The playlist switch and the playlist stats sensors live on the same device and
+share the same lookup, availability, and running-state logic. Keeping that in
+one place means the snapshot key names (``scene_playlists`` / ``playlist_state``)
+and the "is this the active playlist?" match only ever live once, so the switch
+and sensors can't silently drift apart.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from homeassistant.helpers.update_coordinator import CoordinatorEntity
+
+from .const import DOMAIN
+from .coordinator import LedGrabCoordinator
+
+
+class LedGrabPlaylistEntity(CoordinatorEntity):
+    """Common plumbing for entities attached to a scene-playlist device."""
+
+    _attr_has_entity_name = True
+
+    def __init__(
+        self,
+        coordinator: LedGrabCoordinator,
+        playlist_id: str,
+        entry_id: str,
+    ) -> None:
+        super().__init__(coordinator)
+        self._playlist_id = playlist_id
+        self._entry_id = entry_id
+
+    @property
+    def device_info(self) -> dict[str, Any]:
+        return {"identifiers": {(DOMAIN, self._playlist_id)}}
+
+    @property
+    def available(self) -> bool:
+        return self._get_playlist() is not None
+
+    def _get_playlist(self) -> dict[str, Any] | None:
+        if not self.coordinator.data:
+            return None
+        for playlist in self.coordinator.data.get("scene_playlists", []):
+            if playlist.get("id") == self._playlist_id:
+                return playlist
+        return None
+
+    def _running_state(self) -> dict[str, Any] | None:
+        """Return the global cycling state if this playlist is the active one."""
+        if not self.coordinator.data:
+            return None
+        state = self.coordinator.data.get("playlist_state") or {}
+        if state.get("is_running") and state.get("playlist_id") == self._playlist_id:
+            return state
+        return None