Polymorphism Phase 2 + remove unused gamma/saturation fields

ProcessorManager: replace all isinstance checks with property-based
dispatch via base TargetProcessor (device_id, led_client,
get_display_index, update_device, update_calibration).

Remove gamma/saturation from ProcessingSettings, ColorCorrection
schema, serialization, and migration — these were never used in the
processing pipeline and are handled by postprocessing template filters.
Delete dead apply_color_correction() function.

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

server/src/wled_controller/storage/wled_picture_target.py

@@ -59,8 +59,6 @@ class WledPictureTarget(PictureTarget):
             "display_index": self.settings.display_index,
             "fps": self.settings.fps,
             "brightness": self.settings.brightness,
-            "gamma": self.settings.gamma,
-            "saturation": self.settings.saturation,
             "smoothing": self.settings.smoothing,
             "interpolation_mode": self.settings.interpolation_mode,
             "standby_interval": self.settings.standby_interval,
@@ -78,8 +76,6 @@ class WledPictureTarget(PictureTarget):
             display_index=settings_data.get("display_index", 0),
             fps=settings_data.get("fps", 30),
             brightness=settings_data.get("brightness", 1.0),
-            gamma=settings_data.get("gamma", 2.2),
-            saturation=settings_data.get("saturation", 1.0),
             smoothing=settings_data.get("smoothing", 0.3),
             interpolation_mode=settings_data.get("interpolation_mode", "average"),
             standby_interval=settings_data.get("standby_interval", 1.0),