feat(snapshot): include scene playlists + cycling state in snapshot

The aggregated /api/v1/snapshot poll now emits a `scene_playlists` section
(each playlist with its `is_running` flag) plus a companion `playlist_state`
key carrying the single global cycling state (running playlist, current index/
preset, dwell) — so the HA-coordinator and other low-overhead pollers get
playlist state in the same round trip as scenes/targets, matching the other
entity sections. Gated by the `scene_playlists` include-section like the rest.

Reuses the existing list_scene_playlists handler; snapshot route tests updated.

server/tests/api/routes/test_snapshot_routes.py

@@ -33,6 +33,8 @@ _TOP_LEVEL_KEYS = (
     "css_sources",
     "value_sources",
     "scene_presets",
+    "scene_playlists",
+    "playlist_state",
     "sync_clocks",
     "system",
 )
@@ -56,6 +58,21 @@ def client(test_config, monkeypatch):
     value_store.get_all_sources.return_value = []
     preset_store = MagicMock()
     preset_store.get_all_presets.return_value = []
+    playlist_store = MagicMock()
+    playlist_store.get_all_playlists.return_value = []
+    playlist_engine = MagicMock()
+    playlist_engine.get_running_playlist_id.return_value = None
+    playlist_engine.get_state.return_value = {
+        "is_running": False,
+        "playlist_id": None,
+        "playlist_name": None,
+        "current_index": 0,
+        "item_count": 0,
+        "current_preset_id": None,
+        "started_at": None,
+        "step_started_at": None,
+        "step_duration": 0.0,
+    }
     clock_store = MagicMock()
     clock_store.get_all_clocks.return_value = []
     clock_manager = MagicMock()
@@ -74,6 +91,8 @@ def client(test_config, monkeypatch):
     app.dependency_overrides[deps.get_color_strip_store] = lambda: css_store
     app.dependency_overrides[deps.get_value_source_store] = lambda: value_store
     app.dependency_overrides[deps.get_scene_preset_store] = lambda: preset_store
+    app.dependency_overrides[deps.get_scene_playlist_store] = lambda: playlist_store
+    app.dependency_overrides[deps.get_playlist_engine] = lambda: playlist_engine
     app.dependency_overrides[deps.get_sync_clock_store] = lambda: clock_store
     app.dependency_overrides[deps.get_sync_clock_manager] = lambda: clock_manager
     app.dependency_overrides[deps.get_processor_manager] = lambda: manager
@@ -97,12 +116,16 @@ def test_snapshot_returns_all_sections(client):
         "css_sources",
         "value_sources",
         "scene_presets",
+        "scene_playlists",
         "sync_clocks",
     ):
         assert data[list_key] == []
     for dict_key in ("target_states", "target_metrics", "device_brightness"):
         assert data[dict_key] == {}
 
+    # The single global cycling state rides along with the playlist list.
+    assert data["playlist_state"]["is_running"] is False
+
 
 def test_snapshot_system_block_has_health_version(client):
     data = client.get("/api/v1/snapshot", headers=_AUTH).json()