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/circuit.py

@@ -19,6 +19,14 @@ class Circuit:
     locality: 'Locality'
     layouts: dict[str, TrackLayout]
 
+    def to_dict(self) -> dict:
+        return {
+            "slug": self.slug,
+            "name": self.name,
+            "urls": {"wikipedia": self.urls.wikipedia, "fandom": self.urls.fandom},
+            "layouts": {k: v.to_dict() for k, v in self.layouts.items()},
+        }
+
     @classmethod
     def from_dict(cls, locality: 'Locality', slug: str, data: dict) -> 'Circuit':
         circuit = cls(