Add 1920x1080 session thumbs with track overlay + episode YAML entries

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

cdn-api.py

@@ -100,6 +100,25 @@ async def get_season_poster(season: int):
     )
 
 
+@app.get("/posters/session/{grand_prix_name}/{season}/{session_slug}.png")
+async def get_session_thumb(grand_prix_name: str, season: int, session_slug: str):
+    """Plex episode-thumb (1920x1080) for a specific session of a race weekend.
+
+    `session_slug` is one of: fp1, fp2, fp3, pre-qualifying, qualifying,
+    post-qualifying, pre-race, race, post-race, highlights, pre-sprint,
+    sprint, post-sprint, pre-sprint-qualifying, sprint-qualifying,
+    post-sprint-qualifying, bonus.
+    """
+    png = posters_service.render_session_thumb(grand_prix_name, season, session_slug)
+    if not png:
+        raise HTTPException(status_code=404, detail=f"No session thumb for {grand_prix_name} {season} {session_slug}")
+    return FastAPIResponse(
+        content=png,
+        media_type="image/png",
+        headers={"Cache-Control": "public, max-age=31536000"},
+    )
+
+
 @app.get("/kometa/formula-1-metadata.yml")
 async def get_kometa_metadata():
     """Generate a Kometa-compatible metadata YAML for the Formula 1 library.