Add brightness control to Key Colors targets with HAOS integration

Adds brightness (0.0-1.0) to KeyColorsSettings, applies scaling in the
KC processing pipeline after smoothing, exposes a slider on the WebUI
target card, and creates a HA number entity for KC target brightness.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes/picture_targets.py

@@ -106,6 +106,7 @@ def _kc_settings_to_schema(settings: KeyColorsSettings) -> KeyColorsSettingsSche
         interpolation_mode=settings.interpolation_mode,
         smoothing=settings.smoothing,
         pattern_template_id=settings.pattern_template_id,
+        brightness=settings.brightness,
     )
 
 
@@ -116,6 +117,7 @@ def _kc_schema_to_settings(schema: KeyColorsSettingsSchema) -> KeyColorsSettings
         interpolation_mode=schema.interpolation_mode,
         smoothing=schema.smoothing,
         pattern_template_id=schema.pattern_template_id,
+        brightness=schema.brightness,
     )