feat(mqtt): multi-broker MQTT + Zigbee2MQTT light target

- New Z2MLightOutputTarget storage, processor, editor and routes for
  Zigbee2MQTT light entities (shares the HA-Light editor UI via the new
  light-target-editor module)
- Replace global MQTTService/MQTTConfig with per-source MQTTManager +
  MQTTRuntime; thread mqtt_source_id through Z2M targets, DIY MQTT
  devices, and the automation engine
- Migrate legacy single-broker YAML/env config to a "Default Broker"
  MQTTSource on startup (core/mqtt/legacy_migration.py) and drop the
  obsolete core/mqtt/mqtt_service.py
- Refresh /api/v1/system integration status to surface every MQTT source
- Extract shared light-target editor and refactor OutputTargetStore +
  output_targets routes around typed factories / auto-registry
- Modal CSS polish, locale strings, and storage/bindable test coverage

server/tests/core/test_ha_light_target_processor.py

@@ -340,6 +340,89 @@ async def test_brightness_clamped_at_255():
     assert turn_on.service_data["brightness"] == 255
 
 
+@pytest.mark.asyncio
+async def test_turn_off_lights_when_idle_borrows_runtime_from_manager():
+    """Manual turn-off must work even when the processor isn't running."""
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+
+    proc = HALightTargetProcessor(
+        target_id="t_off_idle",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_x",
+        light_mappings=[_mapping("light.a"), _mapping("light.b"), _mapping("light.a")],
+        ctx=_make_ctx(ha_manager=ha_mgr),
+    )
+
+    count = await proc.turn_off_lights()
+
+    assert count == 2  # duplicates collapsed
+    assert ha_mgr.acquired_for == ["ha_1"]
+    assert ha_mgr.released_for == ["ha_1"]
+    services = [(c.service, c.target["entity_id"]) for c in runtime.calls]
+    assert ("turn_off", "light.a") in services
+    assert ("turn_off", "light.b") in services
+    assert all(c.service == "turn_off" for c in runtime.calls)
+
+
+@pytest.mark.asyncio
+async def test_turn_off_lights_while_running_uses_existing_runtime():
+    """If running, turn_off must reuse self._ha_runtime — not borrow another."""
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+    color_stream = _FakeColorStream((10, 20, 30))
+
+    proc = HALightTargetProcessor(
+        target_id="t_off_run",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_x",
+        light_mappings=[_mapping("light.a")],
+        color_tolerance=0,
+        ctx=_make_ctx(ha_manager=ha_mgr, vs_manager=_FakeVSManager(color_stream)),
+    )
+
+    await proc.start()
+    try:
+        # One acquire from start(); turn_off_lights must NOT add another.
+        before_acquires = list(ha_mgr.acquired_for)
+        before_releases = list(ha_mgr.released_for)
+        count = await proc.turn_off_lights()
+    finally:
+        await proc.stop()
+
+    assert count == 1
+    # No extra acquire/release pairs while running.
+    assert len(ha_mgr.acquired_for) == len(before_acquires)
+    assert len(ha_mgr.released_for) == len(before_releases) + 1  # +1 from stop()
+    assert any(
+        c.service == "turn_off" and c.target["entity_id"] == "light.a" for c in runtime.calls
+    )
+
+
+@pytest.mark.asyncio
+async def test_turn_off_lights_no_mappings_returns_zero():
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+
+    proc = HALightTargetProcessor(
+        target_id="t_off_empty",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_x",
+        light_mappings=[],
+        ctx=_make_ctx(ha_manager=ha_mgr),
+    )
+
+    count = await proc.turn_off_lights()
+
+    assert count == 0
+    # No need to acquire when there's nothing to do.
+    assert ha_mgr.acquired_for == []
+    assert runtime.calls == []
+
+
 @pytest.mark.asyncio
 async def test_get_state_reports_source_kind():
     runtime = _FakeHARuntime()