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>

tests/test_sepa.py

@@ -178,11 +178,11 @@ def test_debit_mail_is_thunderbird_draft(tmp_path):
     debit = pending_direct_debits(repository, due_until=date(2026, 12, 31))[0][0]
 
     content = debit_mail_bytes(
-        recipient=member.email,
-        first_name=member.first_name,
+        repository,
+        member=member,
         debit=debit,
         collection_date=date(2026, 8, 3),
-        creditor_id="DE98ZZZ09999999999",
+        organization={"name": "C3MA", "creditor_id": "DE98ZZZ09999999999"},
         sender_name="Verwaltung C3MA",
         sender_email="verwaltung@example.org",
         signature="Der Vorstand",