feat: add card-renderer internal service (v1)

- New card-renderer FastAPI service (Python 3.11 + Pillow)
  - GET /health, GET /templates
  - POST /render (URL input)
  - POST /render/file (multipart upload)
  - POST /render/meta (dry-run layout metadata)
  - nova-artwork-v1 template: cover crop, gradient overlay, text, logo
  - SSRF-safe async image fetch with redirect validation
  - Smart center cover crop isolated for future YOLO focal-point support
  - Graceful font/logo fallback when assets are absent

- docker-compose.yml: add card-renderer service + healthcheck;
  extend gateway with CARD_RENDERER_URL and depends_on

- gateway/main.py: proxy endpoints under /cards/*
  - GET  /cards/templates
  - POST /cards/render
  - POST /cards/render/file
  - POST /cards/render/meta
  All protected by existing APIKeyMiddleware

card-renderer/Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libgl1 \
+    libglib2.0-0 \
+  && rm -rf /var/lib/apt/lists/*
+
+COPY card-renderer/requirements.txt /app/requirements.txt
+RUN pip install --no-cache-dir -r /app/requirements.txt
+
+COPY card-renderer /app
+
+ENV PYTHONUNBUFFERED=1
+
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]