Push a state snapshot only for events that change it

The hub sent every open tab a full snapshot on every StateEvent, LED writes
included - placing and removing a figure was 13 snapshots, 9 of them for LED
changes the snapshot does not even contain. List the events that matter, and make
a test fail when a new StateEvent is sorted into neither list.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/tests/test_web.py

@@ -21,15 +21,20 @@ from fastapi import FastAPI
 
 from musicmouse.app import ParentTimer
 from musicmouse.config import WebConfig, load_config
+from musicmouse.effects import OFF
 from musicmouse.events import (
     CancelTimerRequested,
     ClockStateChanged,
+    LedEffectChanged,
     SetTimerRequested,
+    StateEvent,
     ToggleClockRequested,
     ToggleScreenRequested,
 )
+from musicmouse.hardware import LedZone
 from musicmouse.library.analysis import BeatGrid, TrackCurves
 from musicmouse.library.models import track_key
+from musicmouse.services.web.hub import IGNORED_EVENTS, SNAPSHOT_EVENTS
 from musicmouse.services.web.service import build_app
 from musicmouse.simulator.harness import Simulation, build_simulation
 from tests.conftest import VALID_CONFIG, write_config
@@ -442,6 +447,30 @@ async def test_a_state_change_reaches_every_client(api: FastAPI, sim: Simulation
             assert message["state"]["active_figure"] == "fuchs"
 
 
+async def test_led_writes_are_not_pushed_to_the_clients(api: FastAPI, sim: Simulation) -> None:
+    """The snapshot has no LED state, so an LED write is no reason to send one."""
+    async with websocket_connect(api, "/api/ws") as socket:
+        await socket.next_json()
+
+        await sim.bus.emit_and_wait(
+            LedEffectChanged(zone=LedZone.RING, effect=OFF(), origin="mqtt")
+        )
+
+        assert socket.from_app.empty()
+
+
+def test_every_state_event_is_either_pushed_or_deliberately_ignored() -> None:
+    """A new StateEvent has to be sorted into SNAPSHOT_EVENTS or IGNORED_EVENTS."""
+
+    def subclasses(kind: type) -> set[type]:
+        found = set(kind.__subclasses__())
+        for sub in list(found):
+            found |= subclasses(sub)
+        return found
+
+    assert subclasses(StateEvent) == set(SNAPSHOT_EVENTS) | set(IGNORED_EVENTS)
+
+
 async def test_switching_albums_while_playing_reaches_every_client(
     api: FastAPI, client: Client, sim: Simulation
 ) -> None: