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/static/locales/en.json

@@ -298,5 +298,45 @@
   "streams.image_source.hint": "Enter a URL (http/https) or local file path to an image",
   "streams.validate_image.validating": "Validating...",
   "streams.validate_image.valid": "Image accessible",
-  "streams.validate_image.invalid": "Image not accessible"
+  "streams.validate_image.invalid": "Image not accessible",
+  "targets.title": "⚡ Targets",
+  "targets.description": "Targets bridge picture sources to output devices. Each target references a device and a source, with its own processing settings.",
+  "targets.add": "Add Target",
+  "targets.edit": "Edit Target",
+  "targets.loading": "Loading targets...",
+  "targets.none": "No targets configured",
+  "targets.failed": "Failed to load targets",
+  "targets.name": "Target Name:",
+  "targets.name.placeholder": "My Target",
+  "targets.device": "Device:",
+  "targets.device.hint": "Which WLED device to send LED data to",
+  "targets.device.none": "-- Select a device --",
+  "targets.source": "Source:",
+  "targets.source.hint": "Which picture source to capture and process",
+  "targets.source.none": "-- No source assigned --",
+  "targets.border_width": "Border Width (px):",
+  "targets.border_width.hint": "How many pixels from the screen edge to sample for LED colors (1-100)",
+  "targets.interpolation": "Interpolation Mode:",
+  "targets.interpolation.hint": "How to calculate LED color from sampled pixels",
+  "targets.interpolation.average": "Average",
+  "targets.interpolation.median": "Median",
+  "targets.interpolation.dominant": "Dominant",
+  "targets.smoothing": "Smoothing:",
+  "targets.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "targets.created": "Target created successfully",
+  "targets.updated": "Target updated successfully",
+  "targets.deleted": "Target deleted successfully",
+  "targets.delete.confirm": "Are you sure you want to delete this target?",
+  "targets.error.load": "Failed to load targets",
+  "targets.error.required": "Please fill in all required fields",
+  "targets.error.delete": "Failed to delete target",
+  "targets.button.start": "Start",
+  "targets.button.stop": "Stop",
+  "targets.status.processing": "Processing",
+  "targets.status.idle": "Idle",
+  "targets.status.error": "Error",
+  "targets.metrics.actual_fps": "Actual FPS",
+  "targets.metrics.target_fps": "Target FPS",
+  "targets.metrics.frames": "Frames",
+  "targets.metrics.errors": "Errors"
 }