fix(deploy): put landing on proxy's network + runtime DNS resolve

The landing service had no networks: key, so it joined the auto 'default'
network while proxy was only on spaceshell-network + webproxy — they shared no
network, so proxy_pass to 'landing' couldn't resolve. With a static
upstream{ server landing:80 } nginx fails to boot on an unresolvable name and
restart-loops, so the proxy flapped (page intermittently up/down). Fixes:
- landing now joins spaceshell-network (shared with proxy).
- proxy.conf resolves 'landing' at request time via Docker DNS (127.0.0.11)
  using a variable proxy_pass, so nginx starts even if landing is briefly down.
nginx -t passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/docker-compose.yaml

@@ -8,6 +8,9 @@ services:
     restart: unless-stopped
     expose:
       - "80"
+    networks:
+      # Must share a network with `proxy`, else proxy_pass to `landing` can't resolve.
+      - spaceshell-network
 
   proxy:
     image: nginx:1.27-alpine