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.

requirements.txt

@@ -0,0 +1,10 @@
+blinker==1.6.2
+click==8.1.7
+Flask==2.3.3
+gunicorn==21.2.0
+itsdangerous==2.1.2
+Jinja2==3.1.2
+MarkupSafe==2.1.3
+prometheus-client==0.19.0
+prometheus-flask-exporter==0.23.0
+Werkzeug==2.3.7