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(

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(

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(

models/track_layout.py

@@ -27,6 +27,13 @@ class TrackLayout:
             circuit=circuit
         )
 
+    def to_dict(self) -> dict:
+        return {
+            "slug": self.slug,
+            "description": self.description,
+            "imageUrl": self.imageUrl,
+        }
+
     @property
     def coordinates(self) -> list[tuple[float, float]]:
         return self._geo_data.features[0].geometry.coordinates