fix(scenes): sync brightness value-source change to live processor

apply_scene_state computed brightness_changed = "brightness" in changed, but
the change dict only ever uses the key "brightness_value_source_id", so the
branch was dead and a running target's brightness source was never live-synced
on scene activation (it only took effect after a restart). Check the correct
key.

server/src/ledgrab/core/scenes/scene_activator.py

@@ -93,7 +93,7 @@ async def apply_scene_state(
                 proc = processor_manager.get_processor(ts.target_id)
                 if proc and proc.is_running:
                     css_changed = "color_strip_source_id" in changed
-                    brightness_changed = "brightness" in changed
+                    brightness_changed = "brightness_value_source_id" in changed
                     settings_changed = "fps" in changed
                     if css_changed:
                         target.sync_with_manager(