Fix circular reference recursion in JSON serialization

Dataclass models have parent back-references (TrackLayout→Circuit→
Locality→Country) causing infinite recursion in dataclasses.asdict().
Add to_dict() methods that exclude parent refs, use them in API endpoints.

models/country.py

@@ -7,6 +7,13 @@ class Country:
     name: str
     localities: dict[str, Locality]
 
+    def to_dict(self) -> dict:
+        return {
+            "slug": self.slug,
+            "name": self.name,
+            "localities": {k: v.to_dict() for k, v in self.localities.items()},
+        }
+
     @classmethod
     def from_dict(cls, slug: str, data: dict):
         country = cls(