Introduce Picture Targets to separate processing from devices

Add PictureTarget entity that bridges PictureSource to output device,
separating processing settings from device connection/calibration state.
This enables future target types (Art-Net, E1.31) and cleanly decouples
"what to stream" from "where to stream."

- Add PictureTarget/WledPictureTarget dataclasses and storage
- Split ProcessorManager into DeviceState (health) + TargetState (processing)
- Add /api/v1/picture-targets endpoints (CRUD, start/stop, settings, metrics)
- Simplify device API (remove processing/settings/metrics endpoints)
- Auto-migrate existing device settings to picture targets on first startup
- Add Targets tab to WebUI with target cards and editor modal
- Add en/ru locale keys for targets UI

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

server/src/wled_controller/config.py

@@ -57,6 +57,7 @@ class StorageConfig(BaseSettings):
     templates_file: str = "data/capture_templates.json"
     postprocessing_templates_file: str = "data/postprocessing_templates.json"
     picture_sources_file: str = "data/picture_sources.json"
+    picture_targets_file: str = "data/picture_targets.json"
 
 
 class LoggingConfig(BaseSettings):