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/locality.py

@@ -12,6 +12,13 @@ class Locality:
     country: 'Country'
     circuits: dict[str, Circuit]
 
+    def to_dict(self) -> dict:
+        return {
+            "slug": self.slug,
+            "name": self.name,
+            "circuits": {k: v.to_dict() for k, v in self.circuits.items()},
+        }
+
     @classmethod
     def from_dict(cls, country: 'Country', slug: str, data: dict):
         locality = cls(