Refactor core/ into logical sub-packages and split filter files

Reorganize the flat core/ directory (17 files) into three sub-packages:
- core/devices/ — LED device communication (led_client, wled/adalight clients, providers, DDP)
- core/processing/ — target processing pipeline (processor_manager, target processors, live streams, settings)
- core/capture/ — screen capture & calibration (screen_capture, calibration, pixel_processor, overlay)

Also split the monolithic filters/builtin.py (460 lines, 8 filters) into
individual files: brightness, saturation, gamma, downscaler, pixelate,
auto_crop, flip, color_correction.

Includes the ProcessorManager refactor from target-centric architecture:
ProcessorManager slimmed from ~1600 to ~490 lines with unified
_processors dict replacing duplicate _targets/_kc_targets dicts.

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

server/src/wled_controller/api/schemas/devices.py

@@ -14,6 +14,7 @@ class DeviceCreate(BaseModel):
     device_type: str = Field(default="wled", description="LED device type (e.g., wled, adalight)")
     led_count: Optional[int] = Field(None, ge=1, le=10000, description="Number of LEDs (required for adalight)")
     baud_rate: Optional[int] = Field(None, description="Serial baud rate (for adalight devices)")
+    auto_shutdown: Optional[bool] = Field(default=None, description="Turn off device when server stops (defaults to true for adalight)")
 
 
 class DeviceUpdate(BaseModel):
@@ -24,6 +25,7 @@ class DeviceUpdate(BaseModel):
     enabled: Optional[bool] = Field(None, description="Whether device is enabled")
     led_count: Optional[int] = Field(None, ge=1, le=10000, description="Number of LEDs (for devices with manual_led_count capability)")
     baud_rate: Optional[int] = Field(None, description="Serial baud rate (for adalight devices)")
+    auto_shutdown: Optional[bool] = Field(None, description="Turn off device when server stops")
 
 
 class Calibration(BaseModel):
@@ -90,6 +92,7 @@ class DeviceResponse(BaseModel):
     led_count: int = Field(description="Total number of LEDs")
     enabled: bool = Field(description="Whether device is enabled")
     baud_rate: Optional[int] = Field(None, description="Serial baud rate")
+    auto_shutdown: bool = Field(default=False, description="Turn off device when server stops")
     capabilities: List[str] = Field(default_factory=list, description="Device type capabilities")
     calibration: Optional[Calibration] = Field(None, description="Calibration configuration")
     created_at: datetime = Field(description="Creation timestamp")