Add the example stack payloads

The files copied to /srv/stacks/<stack>/ verbatim: compose files, vhosts,
Prometheus configuration, the webapp image source. Payloads are data, never
templated and never linted as Ansible content.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

server/edge/reverse-proxy/proxy-data/conf/max-body-size.conf

@@ -0,0 +1,3 @@
+# Applies to every vhost. nginx defaults to 1M, which silently truncates ordinary
+# file uploads.
+client_max_body_size 100M;