infra: self-heal nginx after unordered stack restarts

A concurrent restart (docker compose restart / Docker Desktop) can start
nginx before the backend's DNS entry exists; "host not found in upstream"
is fatal at config load and left the gateway dead (502s). restart:
unless-stopped retries until dependencies are up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BFRyKxkKEjfAgpXygzoNiD

docker-compose.yml

@@ -137,6 +137,10 @@ services:
 
   nginx:
     image: nginx:1.27-alpine
+    # Restart on failure: a concurrent stack restart can start nginx before the
+    # backend's DNS entry exists ("host not found in upstream"), which is fatal
+    # at config load — retrying once dependencies are up self-heals the gateway.
+    restart: unless-stopped
     volumes:
       - ./infra/nginx/nginx.conf:/etc/nginx/nginx.conf:ro
     depends_on: