#1 Add Plex-ready race + season poster endpoints

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.