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/api/__init__.py

@@ -7,6 +7,7 @@ from .routes.devices import router as devices_router
 from .routes.templates import router as templates_router
 from .routes.postprocessing import router as postprocessing_router
 from .routes.picture_sources import router as picture_sources_router
+from .routes.picture_targets import router as picture_targets_router
 
 router = APIRouter()
 router.include_router(system_router)
@@ -14,5 +15,6 @@ router.include_router(devices_router)
 router.include_router(templates_router)
 router.include_router(postprocessing_router)
 router.include_router(picture_sources_router)
+router.include_router(picture_targets_router)
 
 __all__ = ["router"]