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.

cdn-api.py

@@ -67,7 +67,7 @@ async def root():
 async def get_countries():
     """Get list of all available countries with their slugs"""
     return {
-        "countries": circuit_service.get_countries_list()
+        "countries": [c.to_dict() for c in circuit_service.get_countries_list()]
     }
 
 @app.get("/circuits/{country_slug}")
@@ -76,7 +76,7 @@ async def get_cities(country_slug: str):
     cities = circuit_service.get_localities_list(country_slug)
     if not cities:
         raise HTTPException(status_code=404, detail="Country not found")
-    return {"cities": cities}
+    return {"cities": [c.to_dict() for c in cities]}
 
 @app.get("/circuits/{country_slug}/{city_slug}")
 async def get_circuits(country_slug: str, city_slug: str):
@@ -84,7 +84,7 @@ async def get_circuits(country_slug: str, city_slug: str):
     circuits = circuit_service.get_circuits_list(country_slug, city_slug)
     if not circuits:
         raise HTTPException(status_code=404, detail="City not found")
-    return {"circuits": circuits}
+    return {"circuits": [c.to_dict() for c in circuits]}
 
 @app.get("/circuits/{country_slug}/{city_slug}/{circuit_slug}")
 async def get_circuit_details(country_slug: str, city_slug: str, circuit_slug: str):
@@ -92,7 +92,7 @@ async def get_circuit_details(country_slug: str, city_slug: str, circuit_slug: s
     circuit_details = circuit_service.get_circuit_details(country_slug, city_slug, circuit_slug)
     if not circuit_details:
         raise HTTPException(status_code=404, detail="Circuit not found")
-    return circuit_details
+    return circuit_details.to_dict()
 
 @app.get("/circuits/{country_slug}/{city_slug}/{circuit_slug}/layout/{layout_slug}")
 async def get_layout_details(country_slug: str, city_slug: str, circuit_slug: str, layout_slug: str):
@@ -100,7 +100,7 @@ async def get_layout_details(country_slug: str, city_slug: str, circuit_slug: st
     layout_details = circuit_service.get_layout_details(country_slug, city_slug, circuit_slug, layout_slug)
     if not layout_details:
         raise HTTPException(status_code=404, detail="Layout not found")
-    return layout_details
+    return layout_details.to_dict()
 
 @app.get("/tracks/{country_slug}/{city_slug}/{circuit_slug}/year/{year}/{image_format}")
 async def get_track_by_year(