Render every outgoing e-mail from an editable text template

The wording of the dunning and SEPA pre-notification mails was hard-coded in
Python, so adjusting a single sentence required a new release. Both texts now
live in plain text templates that ship as defaults, are copied into the store's
templates/mail/ directory on first start and can be edited there or under
Optionen -> E-Mail-Vorlagen; an existing file is never overwritten and a deleted
one is restored from the shipped default.

A template carries its subject in the first line and the body after a blank
line. Placeholders use the same {{ ... }} syntax as the document templates and
share their member/organization values, so a name means the same thing in a
letter and in the mail that carries it. Unknown placeholders are rejected while
editing instead of during a send run, a line holding nothing but placeholders
that render empty is dropped, and {{#claims}} ... {{/claims}} repeats per entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pyproject.toml

@@ -42,6 +42,7 @@ ccma = [
     "assets/CHANGELOG.json",
     "assets/splash.png",
     "assets/templates/*",
+    "assets/mail_templates/*",
     "assets/themes/forest/**/*",
     "assets/themes/forest/*",
 ]