GET /posters/race/{grand_prix_name}/{season}.png
GET /posters/season/{season}.png

1000x1500 PNGs composed with Pillow + cairosvg:
checkered-flag strips top/bottom, F1 wordmark, round/year ribbon,
GP name, rasterised track outline (white), race date. Season poster
is wordmark + giant year + 'WORLD CHAMPIONSHIP'. Cached under
cache/posters/ so cold renders happen once.

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.