Add Picture Streams architecture with postprocessing templates and stream test UI

Introduce Picture Stream abstraction that separates the capture pipeline into
composable layers: raw streams (display + capture engine + FPS) and processed
streams (source stream + postprocessing template). Devices reference a picture
stream instead of managing individual capture settings.

- Add PictureStream and PostprocessingTemplate data models and stores
- Add CRUD API endpoints for picture streams and postprocessing templates
- Add stream chain resolution in ProcessorManager for start_processing
- Add picture stream test endpoint with postprocessing preview support
- Add Stream Settings modal with border_width and interpolation_mode controls
- Add stream test modal with capture preview and performance metrics
- Add full frontend: Picture Streams tab, Processing Templates tab, stream
  selector on device cards, test buttons on stream cards
- Add localization keys for all new features (en, ru)
- Migrate existing devices to picture streams on startup

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

server/src/wled_controller/storage/device_store.py

@@ -30,7 +30,8 @@ class Device:
         enabled: bool = True,
         settings: Optional[ProcessingSettings] = None,
         calibration: Optional[CalibrationConfig] = None,
-        capture_template_id: str = "tpl_mss_default",
+        capture_template_id: str = "",
+        picture_stream_id: str = "",
         created_at: Optional[datetime] = None,
         updated_at: Optional[datetime] = None,
     ):
@@ -44,7 +45,8 @@ class Device:
             enabled: Whether device is enabled
             settings: Processing settings
             calibration: Calibration configuration
-            capture_template_id: ID of assigned capture template
+            capture_template_id: ID of assigned capture template (legacy, use picture_stream_id)
+            picture_stream_id: ID of assigned picture stream
             created_at: Creation timestamp
             updated_at: Last update timestamp
         """
@@ -56,6 +58,7 @@ class Device:
         self.settings = settings or ProcessingSettings()
         self.calibration = calibration or create_default_calibration(led_count)
         self.capture_template_id = capture_template_id
+        self.picture_stream_id = picture_stream_id
         self.created_at = created_at or datetime.utcnow()
         self.updated_at = updated_at or datetime.utcnow()
 
@@ -84,6 +87,7 @@ class Device:
             },
             "calibration": calibration_to_dict(self.calibration),
             "capture_template_id": self.capture_template_id,
+            "picture_stream_id": self.picture_stream_id,
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),
         }
@@ -121,11 +125,8 @@ class Device:
             else create_default_calibration(data["led_count"])
         )
 
-        # Migration: assign default MSS template if no template set
-        capture_template_id = data.get("capture_template_id")
-        if not capture_template_id:
-            capture_template_id = "tpl_mss_default"
-            logger.info(f"Migrating device {data['id']} to default MSS template")
+        capture_template_id = data.get("capture_template_id", "")
+        picture_stream_id = data.get("picture_stream_id", "")
 
         return cls(
             device_id=data["id"],
@@ -136,6 +137,7 @@ class Device:
             settings=settings,
             calibration=calibration,
             capture_template_id=capture_template_id,
+            picture_stream_id=picture_stream_id,
             created_at=datetime.fromisoformat(data.get("created_at", datetime.utcnow().isoformat())),
             updated_at=datetime.fromisoformat(data.get("updated_at", datetime.utcnow().isoformat())),
         )
@@ -217,7 +219,8 @@ class DeviceStore:
         led_count: int,
         settings: Optional[ProcessingSettings] = None,
         calibration: Optional[CalibrationConfig] = None,
-        capture_template_id: str = "tpl_mss_default",
+        capture_template_id: str = "",
+        picture_stream_id: str = "",
     ) -> Device:
         """Create a new device.
 
@@ -247,6 +250,7 @@ class DeviceStore:
             settings=settings,
             calibration=calibration,
             capture_template_id=capture_template_id,
+            picture_stream_id=picture_stream_id,
         )
 
         # Store
@@ -285,6 +289,7 @@ class DeviceStore:
         settings: Optional[ProcessingSettings] = None,
         calibration: Optional[CalibrationConfig] = None,
         capture_template_id: Optional[str] = None,
+        picture_stream_id: Optional[str] = None,
     ) -> Device:
         """Update device.
 
@@ -331,6 +336,8 @@ class DeviceStore:
             device.calibration = calibration
         if capture_template_id is not None:
             device.capture_template_id = capture_template_id
+        if picture_stream_id is not None:
+            device.picture_stream_id = picture_stream_id
 
         device.updated_at = datetime.utcnow()