Use season hero photos as poster bg + switch image to Alpine

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.

service/Dockerfile

@@ -1,24 +1,26 @@
-FROM python:3.12-slim
+FROM python:3.12-alpine
 
 WORKDIR /app
 
-# Poster rendering needs:
-#   - libcairo2 (cairosvg ↔ Cairo bindings) for SVG → PNG raster
-#   - fonts-dejavu-core for the DejaVuSans-Bold font used by PostersService
-# These add ~15 MB to the image but keep poster generation in-process and fast.
-RUN apt-get update && \
-    apt-get install -y --no-install-recommends \
-        libcairo2 \
-        fonts-dejavu-core \
-    && rm -rf /var/lib/apt/lists/*
+# Runtime deps:
+#   - cairo provides libcairo so cairosvg can rasterise SVG → PNG.
+#   - ttf-liberation gives Liberation Sans Bold at
+#     /usr/share/fonts/liberation/LiberationSans-Bold.ttf — exactly where Arch
+#     installs it, so dev (shanks) and prod (this container) render with the
+#     same font and the posters come out identical.
+RUN apk add --no-cache \
+        cairo \
+        ttf-liberation
 
+# All pip deps ship musllinux wheels — no build-deps needed.
+# numpy is pulled in by models/geo_json/geometry.py for vector ops at import time.
 RUN pip install --no-cache-dir \
-    fastapi \
-    uvicorn \
-    fastf1 \
-    python-slugify \
-    pillow \
-    cairosvg
+        fastapi \
+        uvicorn \
+        python-slugify \
+        pillow \
+        cairosvg \
+        numpy
 
 COPY cdn-api.py circuits_service.py ergast_service.py posters_service.py ./
 COPY models/ ./models/