perf(deploy): cache the YouTube artwork at the nginx edge

Thumbnail responses are immutable for a month, so nginx can answer repeats
from disk and never wake the web player or Google for them. The cache lives in
its own volume, collapses concurrent misses with a lock and serves stale
copies while it refreshes; THUMBNAIL_CACHE_SIZE sizes it.

deploy/compose.prod.yml

@@ -29,6 +29,7 @@ services:
       # The filter keeps it from eating nginx's own $host / $remote_addr.
       MUSIFY_ROOT_DOMAIN: ${ROOT_DOMAIN}
       MUSIFY_MAX_UPLOAD_SIZE: ${MAX_UPLOAD_SIZE:-1024m}
+      MUSIFY_THUMBNAIL_CACHE_SIZE: ${THUMBNAIL_CACHE_SIZE:-2g}
       NGINX_ENVSUBST_FILTER: ^MUSIFY_
     ports:
       - "80:80"
@@ -37,4 +38,8 @@ services:
       - ./nginx/templates:/etc/nginx/templates:ro
       - ./nginx/snippets:/etc/nginx/snippets:ro
       - ./nginx/certs:/etc/nginx/certs:ro
+      - nginx_cache:/var/cache/nginx
     restart: unless-stopped
+
+volumes:
+  nginx_cache: