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/storage/test_bindable.py

@@ -0,0 +1,76 @@
+"""Tests for BindableFloat / BindableColor update semantics.
+
+The key behaviour pinned here: when ``apply_update`` receives a plain
+primitive (number for float, ``[R,G,B]`` list for color) it must clear
+``source_id`` -- that is how the client signals "unbind, use the static
+value now". Previously the implementation preserved ``source_id`` on
+plain-primitive updates, which silently dropped unbind requests.
+"""
+
+from ledgrab.storage.bindable import BindableColor, BindableFloat
+
+
+class TestBindableFloatApplyUpdate:
+    def test_plain_number_unbinds_when_previously_bound(self):
+        bf = BindableFloat(value=0.5, source_id="vs_abc")
+        updated = bf.apply_update(0.8)
+        assert updated.value == 0.8
+        assert updated.source_id == ""
+        assert not updated.is_bound
+
+    def test_plain_number_leaves_unbound_unbound(self):
+        bf = BindableFloat(value=0.5, source_id="")
+        updated = bf.apply_update(0.8)
+        assert updated.value == 0.8
+        assert updated.source_id == ""
+
+    def test_dict_with_source_id_binds(self):
+        bf = BindableFloat(value=0.5, source_id="")
+        updated = bf.apply_update({"value": 0.8, "source_id": "vs_new"})
+        assert updated.value == 0.8
+        assert updated.source_id == "vs_new"
+        assert updated.is_bound
+
+    def test_dict_with_empty_source_id_unbinds(self):
+        bf = BindableFloat(value=0.5, source_id="vs_abc")
+        updated = bf.apply_update({"value": 0.8, "source_id": ""})
+        assert updated.value == 0.8
+        assert updated.source_id == ""
+
+    def test_none_leaves_unchanged(self):
+        bf = BindableFloat(value=0.5, source_id="vs_abc")
+        updated = bf.apply_update(None)
+        assert updated is bf
+
+
+class TestBindableColorApplyUpdate:
+    def test_plain_list_unbinds_when_previously_bound(self):
+        bc = BindableColor(color=[10, 20, 30], source_id="vs_abc")
+        updated = bc.apply_update([100, 150, 200])
+        assert updated.color == [100, 150, 200]
+        assert updated.source_id == ""
+        assert not updated.is_bound
+
+    def test_plain_list_leaves_unbound_unbound(self):
+        bc = BindableColor(color=[10, 20, 30], source_id="")
+        updated = bc.apply_update([100, 150, 200])
+        assert updated.color == [100, 150, 200]
+        assert updated.source_id == ""
+
+    def test_dict_with_source_id_binds(self):
+        bc = BindableColor(color=[10, 20, 30], source_id="")
+        updated = bc.apply_update({"color": [100, 150, 200], "source_id": "vs_new"})
+        assert updated.color == [100, 150, 200]
+        assert updated.source_id == "vs_new"
+        assert updated.is_bound
+
+    def test_dict_with_empty_source_id_unbinds(self):
+        bc = BindableColor(color=[10, 20, 30], source_id="vs_abc")
+        updated = bc.apply_update({"color": [100, 150, 200], "source_id": ""})
+        assert updated.color == [100, 150, 200]
+        assert updated.source_id == ""
+
+    def test_none_leaves_unchanged(self):
+        bc = BindableColor(color=[10, 20, 30], source_id="vs_abc")
+        updated = bc.apply_update(None)
+        assert updated is bc