Introduce ColorStripSource as first-class entity

Extracts color processing and calibration out of WledPictureTarget into a
new PictureColorStripSource entity, enabling multiple LED targets to share
one capture/processing pipeline.

New entities & processing:
- storage/color_strip_source.py: ColorStripSource + PictureColorStripSource models
- storage/color_strip_store.py: JSON-backed CRUD store (prefix css_)
- core/processing/color_strip_stream.py: ColorStripStream ABC + PictureColorStripStream (runs border-extract → map → smooth → brightness/sat/gamma in background thread)
- core/processing/color_strip_stream_manager.py: ref-counted shared stream manager

Modified storage/processing:
- WledPictureTarget simplified to device_id + color_strip_source_id + standby_interval + state_check_interval
- Device model: calibration field removed
- WledTargetProcessor: acquires ColorStripStream from manager instead of running its own pipeline
- ProcessorManager: wires ColorStripStreamManager into TargetContext

API layer:
- New routes: GET/POST/PUT/DELETE /api/v1/color-strip-sources, PUT calibration/test
- Removed calibration endpoints from /devices
- Updated /picture-targets CRUD for new target structure

Frontend:
- New color-strips.js module with CSS editor modal and card rendering
- Calibration modal extended with CSS mode (css-id hidden field + device picker)
- targets.js: Color Strip Sources section added to LED tab; target editor/card updated
- app.js: imports and window globals for CSS + showCSSCalibration
- en.json / ru.json: color_strip.* and targets.section.color_strips keys added

Data migration runs at startup: existing WledPictureTargets are converted to
reference a new PictureColorStripSource created from their old settings.

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

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

@@ -23,12 +23,18 @@ from .devices import (
     DeviceStateResponse,
     DeviceUpdate,
 )
+from .color_strip_sources import (
+    ColorStripSourceCreate,
+    ColorStripSourceListResponse,
+    ColorStripSourceResponse,
+    ColorStripSourceUpdate,
+    CSSCalibrationTestRequest,
+)
 from .picture_targets import (
     PictureTargetCreate,
     PictureTargetListResponse,
     PictureTargetResponse,
     PictureTargetUpdate,
-    ProcessingSettings,
     TargetMetricsResponse,
     TargetProcessingState,
 )
@@ -89,11 +95,15 @@ __all__ = [
     "DeviceResponse",
     "DeviceStateResponse",
     "DeviceUpdate",
+    "ColorStripSourceCreate",
+    "ColorStripSourceListResponse",
+    "ColorStripSourceResponse",
+    "ColorStripSourceUpdate",
+    "CSSCalibrationTestRequest",
     "PictureTargetCreate",
     "PictureTargetListResponse",
     "PictureTargetResponse",
     "PictureTargetUpdate",
-    "ProcessingSettings",
     "TargetMetricsResponse",
     "TargetProcessingState",
     "EngineInfo",