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/assets/fonts/.gitkeep

@@ -0,0 +1 @@
+# Put Inter-Bold.ttf here