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/api/schemas/__init__.py

@@ -24,7 +24,6 @@ from .devices import (
     DeviceUpdate,
 )
 from .picture_targets import (
-    ColorCorrection,
     PictureTargetCreate,
     PictureTargetListResponse,
     PictureTargetResponse,
@@ -90,7 +89,6 @@ __all__ = [
     "DeviceResponse",
     "DeviceStateResponse",
     "DeviceUpdate",
-    "ColorCorrection",
     "PictureTargetCreate",
     "PictureTargetListResponse",
     "PictureTargetResponse",