#3 Add session thumbs + episode-level url_poster in Kometa YAML

posters_service:
- Standard / sprint episode-index → (label, slug) maps mirroring Kometa's
  F1 builder numbering
- _fetch_map_landscape: aerial tile stitcher in 1920x1080
- render_session_thumb: GP aerial bg + projected track outline + GP name
  + session label with red accent bar
- _draw_track_projected now takes canvas_w / geo_center_y so it can be
  reused for both 2:3 posters and 16:9 thumbs
- render_kometa_metadata now emits an episodes: block per Plex season
  pointing every Kometa-numbered episode at the matching session thumb;
  sprint vs standard format is detected from the Jolpica race entry

cdn-api:
- GET /posters/session/{name}/{year}/{slug}.png returns the thumb