feat(portals): serve each portal with Caddy instead of nginx (refs #166)

nginx resolves a variable `proxy_pass` upstream itself, using only the `resolver`
directive and never the search domains in /etc/resolv.conf. That cost two
workarounds in one script: rewriting the resolver address for rootless podman
(Docker's 127.0.0.11 is wrong there), and injecting a full FQDN so the bare `bff`
name could resolve on Kubernetes at all.

Caddy dials its upstream per request through the system resolver, which reads
nameserver *and* search domains, so `reverse_proxy bff:8080` resolves on every
engine with no per-engine configuration — and it still starts before the BFF
exists and picks up its restarts. Both workarounds are deleted with the script.

Routing uses mutually-exclusive `handle` blocks, not a bare `try_files`: Caddy
sorts rewrites *before* reverse_proxy, so a top-level SPA fallback would rewrite
every API path to /index.html before the proxy saw it.

infra/host-browser.yml

@@ -10,11 +10,11 @@ services:
       KC_HOSTNAME_BACKCHANNEL_DYNAMIC: "true"
   self-service:
     volumes:
-      - ./local-config/self-service.config.json:/usr/share/nginx/html/config.json:ro,z
+      - ./local-config/self-service.config.json:/usr/share/caddy/config.json:ro,z
   behandel:
     volumes:
-      - ./local-config/behandel.config.json:/usr/share/nginx/html/config.json:ro,z
+      - ./local-config/behandel.config.json:/usr/share/caddy/config.json:ro,z
   # beheer is the same medewerker realm as behandel, so it reuses behandel's config verbatim.
   beheer:
     volumes:
-      - ./local-config/behandel.config.json:/usr/share/nginx/html/config.json:ro,z
+      - ./local-config/behandel.config.json:/usr/share/caddy/config.json:ro,z