Add value sources for dynamic brightness control on LED targets

Introduces a new Value Source entity that produces a scalar float (0.0-1.0)
for dynamic brightness modulation. Three subtypes: Static (constant),
Animated (sine/triangle/square/sawtooth waveform), and Audio-reactive
(RMS/peak/beat from mono audio source). Value sources can be optionally
attached to LED targets to control brightness each frame.

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

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

@@ -94,6 +94,7 @@ def _target_to_response(target) -> PictureTargetResponse:
             target_type=target.target_type,
             device_id=target.device_id,
             color_strip_source_id=target.color_strip_source_id,
+            brightness_value_source_id=target.brightness_value_source_id,
             fps=target.fps,
             keepalive_interval=target.keepalive_interval,
             state_check_interval=target.state_check_interval,
@@ -149,6 +150,7 @@ async def create_target(
             target_type=data.target_type,
             device_id=data.device_id,
             color_strip_source_id=data.color_strip_source_id,
+            brightness_value_source_id=data.brightness_value_source_id,
             fps=data.fps,
             keepalive_interval=data.keepalive_interval,
             state_check_interval=data.state_check_interval,
@@ -263,6 +265,7 @@ async def update_target(
             name=data.name,
             device_id=data.device_id,
             color_strip_source_id=data.color_strip_source_id,
+            brightness_value_source_id=data.brightness_value_source_id,
             fps=data.fps,
             keepalive_interval=data.keepalive_interval,
             state_check_interval=data.state_check_interval,
@@ -280,6 +283,7 @@ async def update_target(
                                   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,
+                brightness_vs_changed=data.brightness_value_source_id is not None,
             )
         except ValueError:
             pass