posters_service.render_season_poster() now fetches /dilbert/sports/
formula1/cars/{year}.jpg from the files-api CDN, centre-crops to 2:3,
and composites a vertical dark gradient so the FORMULA 1 / year /
WORLD CHAMPIONSHIP text stays legible over any photo.

Dockerfile:
  - python:3.12-slim → python:3.12-alpine (smaller, no fastf1 leftover)
  - fonts-dejavu-core → ttf-liberation: same font Arch dev ships, so
    posters render byte-identical on dev and in the container.
  - Drop fastf1 (unused in the served code path); add numpy explicitly
    because models/geo_json/geometry.py imports it at module load.