Add min brightness threshold to LED targets

New per-target property: when effective output brightness
(max pixel value × device/source brightness) falls below
the threshold, LEDs turn off completely. Useful for cutting
dim flicker in audio-reactive and ambient setups.

Threshold slider (0–254) in target editor, badge on card,
hot-swap to running processors, persisted in storage.

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

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

@@ -100,6 +100,7 @@ def _target_to_response(target) -> PictureTargetResponse:
             fps=target.fps,
             keepalive_interval=target.keepalive_interval,
             state_check_interval=target.state_check_interval,
+            min_brightness_threshold=target.min_brightness_threshold,
             description=target.description,
             auto_start=target.auto_start,
             created_at=target.created_at,
@@ -159,6 +160,7 @@ async def create_target(
             fps=data.fps,
             keepalive_interval=data.keepalive_interval,
             state_check_interval=data.state_check_interval,
+            min_brightness_threshold=data.min_brightness_threshold,
             picture_source_id=data.picture_source_id,
             key_colors_settings=kc_settings,
             description=data.description,
@@ -276,6 +278,7 @@ async def update_target(
             fps=data.fps,
             keepalive_interval=data.keepalive_interval,
             state_check_interval=data.state_check_interval,
+            min_brightness_threshold=data.min_brightness_threshold,
             key_colors_settings=kc_settings,
             description=data.description,
             auto_start=data.auto_start,
@@ -295,6 +298,7 @@ async def update_target(
                 settings_changed=(data.fps is not None or
                                   data.keepalive_interval is not None or
                                   data.state_check_interval is not None or
+                                  data.min_brightness_threshold is not None or
                                   data.key_colors_settings is not None),
                 css_changed=data.color_strip_source_id is not None,
                 device_changed=data.device_id is not None,