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_mail_templates.py

@@ -0,0 +1,105 @@
+import pytest
+
+from ccma.domain.mail_templates import (
+    MAIL_TEMPLATES,
+    MailTemplate,
+    MailTemplateError,
+    default_mail_template,
+    parse_mail_template,
+    render_mail_template,
+    serialize_mail_template,
+    validate_mail_template,
+)
+from ccma.storage.repository import MemberRepository, RepositoryError
+
+
+def test_subject_header_round_trips_through_file_format() -> None:
+    text = serialize_mail_template("Hallo {{member.first_name}}", "Zeile eins\nZeile zwei")
+
+    template = parse_mail_template(text)
+
+    assert text.startswith("Betreff: Hallo {{member.first_name}}\n\n")
+    assert template == MailTemplate("Hallo {{member.first_name}}", "Zeile eins\nZeile zwei")
+
+
+def test_shipped_defaults_only_use_declared_placeholders() -> None:
+    for spec in MAIL_TEMPLATES:
+        template = default_mail_template(spec.key)
+        validate_mail_template(spec.key, template.subject, template.body)
+
+
+def test_rendering_fills_values_and_expands_repeat_blocks() -> None:
+    template = MailTemplate(
+        "Beitrag für {{member.first_name}}",
+        "Hallo {{member.first_name}},\n"
+        "{{#claims}}- {{claim.title}}: {{claim.amount}}{{/claims}}\n"
+        "Summe: {{claims.total}}",
+    )
+
+    rendered = render_mail_template(
+        template,
+        {"member.first_name": "Ada", "claims.total": "75.00"},
+        {
+            "claims": [
+                {"claim.title": "Aufnahmegebühr", "claim.amount": "15.00"},
+                {"claim.title": "Mitgliedsbeitrag", "claim.amount": "60.00"},
+            ]
+        },
+    )
+
+    assert rendered.subject == "Beitrag für Ada"
+    assert rendered.body == (
+        "Hallo Ada,\n- Aufnahmegebühr: 15.00\n- Mitgliedsbeitrag: 60.00\nSumme: 75.00"
+    )
+
+
+def test_line_holding_only_an_empty_placeholder_is_dropped() -> None:
+    template = MailTemplate("Betreff", "Erste Zeile\n{{reminder.hint}}\nLetzte Zeile")
+
+    assert render_mail_template(template, {"reminder.hint": ""}).body == "Erste Zeile\nLetzte Zeile"
+    assert (
+        render_mail_template(template, {"reminder.hint": "Hinweis: X"}).body
+        == "Erste Zeile\nHinweis: X\nLetzte Zeile"
+    )
+
+
+def test_unknown_placeholder_is_rejected_while_editing() -> None:
+    with pytest.raises(MailTemplateError, match="member.favourite_colour"):
+        validate_mail_template("welcome", "Betreff", "Hallo {{member.favourite_colour}}")
+
+
+def test_store_copies_are_seeded_editable_and_resettable(tmp_path) -> None:
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+    path = repository.mail_templates_root / "willkommen.txt"
+    assert path.is_file()
+
+    repository.save_mail_template("welcome", subject="Moin {{member.first_name}}", body="Kurz und knapp.")
+    assert repository.get_mail_template("welcome") == MailTemplate(
+        "Moin {{member.first_name}}", "Kurz und knapp."
+    )
+    assert "Moin {{member.first_name}}" in path.read_text(encoding="utf-8")
+
+    repository.reset_mail_template("welcome")
+    assert repository.get_mail_template("welcome") == default_mail_template("welcome")
+
+
+def test_missing_template_file_falls_back_to_shipped_default(tmp_path) -> None:
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+    (repository.mail_templates_root / "mahnung.txt").unlink()
+
+    template = repository.get_mail_template("reminder")
+
+    assert template == default_mail_template("reminder")
+    assert (repository.mail_templates_root / "mahnung.txt").is_file()
+
+
+def test_saving_a_broken_template_is_refused(tmp_path) -> None:
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+
+    with pytest.raises(RepositoryError, match="Platzhalter"):
+        repository.save_mail_template("reminder", subject="Mahnung", body="Hallo {{member.nonsense}}")
+    with pytest.raises(RepositoryError, match="Betreff"):
+        repository.save_mail_template("reminder", subject="   ", body="Hallo")