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/app/webapp/.env.example

@@ -0,0 +1,11 @@
+# Copy to .env ON THE HOST, not here. The compose_stack role excludes .env from
+# every sync (stack_exclude), so a .env committed to this repository would be
+# ignored, and a host's .env is never overwritten by a deploy.
+#
+#   ssh deploy@app.example.com
+#   cd /srv/stacks/webapp && cp .env.example .env && $EDITOR .env
+#
+# webapp.yml refuses to deploy if .env is absent on the host, because every
+# variable below is required with no default.
+POSTGRES_PASSWORD=
+SECRET_KEY=