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.

service/Dockerfile

@@ -2,9 +2,25 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-RUN pip install --no-cache-dir fastapi uvicorn fastf1 python-slugify
+# 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/*
 
-COPY cdn-api.py circuits_service.py ergast_service.py ./
+RUN pip install --no-cache-dir \
+    fastapi \
+    uvicorn \
+    fastf1 \
+    python-slugify \
+    pillow \
+    cairosvg
+
+COPY cdn-api.py circuits_service.py ergast_service.py posters_service.py ./
 COPY models/ ./models/
 COPY circuits/ ./circuits/