Encapsulate target-type dispatch via polymorphism (Phase 1)

Replace isinstance checks with polymorphic methods on PictureTarget
hierarchy: register_with_manager, sync_with_manager, update_fields,
and has_picture_source property.

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

server/src/wled_controller/storage/wled_picture_target.py

@@ -16,6 +16,40 @@ class WledPictureTarget(PictureTarget):
     picture_source_id: str = ""
     settings: ProcessingSettings = field(default_factory=ProcessingSettings)
 
+    def register_with_manager(self, manager) -> None:
+        """Register this WLED target with the processor manager."""
+        if self.device_id:
+            manager.add_target(
+                target_id=self.id,
+                device_id=self.device_id,
+                settings=self.settings,
+                picture_source_id=self.picture_source_id,
+            )
+
+    def sync_with_manager(self, manager, *, settings_changed: bool, source_changed: bool, device_changed: bool) -> None:
+        """Push changed fields to the processor manager."""
+        if settings_changed:
+            manager.update_target_settings(self.id, self.settings)
+        if source_changed:
+            manager.update_target_source(self.id, self.picture_source_id)
+        if device_changed:
+            manager.update_target_device(self.id, self.device_id)
+
+    def update_fields(self, *, name=None, device_id=None, picture_source_id=None,
+                      settings=None, key_colors_settings=None, description=None) -> None:
+        """Apply mutable field updates for WLED targets."""
+        super().update_fields(name=name, description=description)
+        if device_id is not None:
+            self.device_id = device_id
+        if picture_source_id is not None:
+            self.picture_source_id = picture_source_id
+        if settings is not None:
+            self.settings = settings
+
+    @property
+    def has_picture_source(self) -> bool:
+        return True
+
     def to_dict(self) -> dict:
         """Convert to dictionary."""
         d = super().to_dict()