feat: extract mail from mp/mail — initial libreshop/mail

Source moved verbatim from mp/mail/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/mail on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

docker-entrypoint.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+if [ "$ENVIRONMENT" = "production" ]; then
+  exec gunicorn -w 4 -b 0.0.0.0:2222 src.app:app
+else
+  exec python3 src/app.py
+fi