Specify the feature branch of LabThings and enable the feature flag.

I've put the feature flag in `openflexure_microscope_server.things.__init__` so that we only need it once, rather than next to every Thing definition.

I decided it was better here than, for example, in the server or cli modules because this way we still get the feature flags even if the Things are served from a different LabThings server.

src/openflexure_microscope_server/things/__init__.py

@@ -9,6 +9,10 @@ from typing import Optional, Self
 
 import labthings_fastapi as lt
 
+# To ensure consistency, we enable the feature flags we want here. This means
+# they will be enabled in any code that imports a `Thing` from this package.
+lt.FEATURE_FLAGS.validate_properties_on_set = True
+
 
 class OFMThing(lt.Thing):
     """A custom LabThings Thing class for the OpenFlexure Microscope."""