Create SIGRef objects when converting dict to Spot object in the spot object's own __post_init__ method rather than in add_spot.py

data/spot.py

@@ -133,6 +133,17 @@ class Spot:
     # The ID the source gave it, if any.
     source_id: str | None = None
 
+    def __post_init__(self):
+        """Normalise fields that don't survive a plain dict to Spot conversion. This is used in the "add spot" API
+        endpoint where the client is submitting JSON, and we want to recreate a full Spot object, including nested
+        objects such as the sig_refs list.."""
+
+        if self.sig_refs:
+            self.sig_refs = [
+                sig_ref if isinstance(sig_ref, SIGRef) else SIGRef(**sig_ref)
+                for sig_ref in self.sig_refs
+            ]
+
     def infer_missing(self, credentials=None):
         """Infer missing parameters where possible"""