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.

apps/beheer/Caddyfile

@@ -0,0 +1,21 @@
+:80 {
+	# Same-origin API: beheerders use the same medewerker realm as behandel (S-15a).
+	# `handle` blocks are mutually exclusive and matched most-specific-first, so the
+	# SPA fallback below can never swallow an API call — unlike a bare `try_files`,
+	# which Caddy sorts *before* reverse_proxy and would rewrite it to /index.html.
+	#
+	# No `resolver` stanza is needed: Caddy dials the upstream per
+	# request through the system resolver, so it starts before the BFF is up, picks up
+	# its restarts, and honours the DNS search domains in /etc/resolv.conf — which is
+	# what lets the bare `bff` name resolve on Kubernetes as well as under compose.
+	handle /beheer/* {
+		reverse_proxy bff:8080
+	}
+
+	# The Angular app. Client-side routing: an unknown path serves index.html.
+	handle {
+		root * /usr/share/caddy
+		try_files {path} /index.html
+		file_server
+	}
+}