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/picture_target.py

@@ -16,6 +16,27 @@ class PictureTarget:
     updated_at: datetime
     description: Optional[str] = None
 
+    def register_with_manager(self, manager) -> None:
+        """Register this target with the processor manager. Subclasses override."""
+        pass
+
+    def sync_with_manager(self, manager, *, settings_changed: bool, source_changed: bool, device_changed: bool) -> None:
+        """Push changed fields to a running processor. Subclasses override."""
+        pass
+
+    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. Base handles common fields; subclasses handle type-specific ones."""
+        if name is not None:
+            self.name = name
+        if description is not None:
+            self.description = description
+
+    @property
+    def has_picture_source(self) -> bool:
+        """Whether this target type uses a picture source."""
+        return False
+
     def to_dict(self) -> dict:
         """Convert to dictionary."""
         return {