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/picture_targets.py

@@ -8,14 +8,6 @@ from pydantic import BaseModel, Field
 from wled_controller.core.processing.processing_settings import DEFAULT_STATE_CHECK_INTERVAL
 
 
-class ColorCorrection(BaseModel):
-    """Color correction settings."""
-
-    gamma: float = Field(default=2.2, description="Gamma correction", ge=0.1, le=5.0)
-    saturation: float = Field(default=1.0, description="Saturation multiplier", ge=0.0, le=2.0)
-    brightness: float = Field(default=1.0, description="Brightness multiplier", ge=0.0, le=1.0)
-
-
 class ProcessingSettings(BaseModel):
     """Processing settings for a picture target."""
 
@@ -29,10 +21,6 @@ class ProcessingSettings(BaseModel):
         default=DEFAULT_STATE_CHECK_INTERVAL, ge=5, le=600,
         description="Seconds between WLED health checks"
     )
-    color_correction: Optional[ColorCorrection] = Field(
-        default_factory=ColorCorrection,
-        description="Color correction settings"
-    )
 
 
 class KeyColorRectangleSchema(BaseModel):