Reject unbalanced repeat markers in mail templates

validate_mail_template() only looked at placeholder names, and "{{#claims}}" is
not a placeholder -- so an unclosed block or a stray "{{/claims}}" passed the
check and rendered as itself: the member would read the marker in their mail.

Repeat markers are now checked structurally: every one names a block the template
actually has, openers and closers pair up in order, blocks do not nest (the
renderer does not support it either), and the subject takes no markers at all.
Each case explains what is wrong and what is missing.

The same check runs before sending, not just before saving: the templates are
plain files in the store and can be edited outside CCMA, where refusing to send
beats mailing a marker. Saving several edited templates now validates all of
them before writing the first, so a mistake in one no longer leaves the others
half-saved.

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

src/ccma/services/mail_composition.py

@@ -5,7 +5,13 @@ from email.message import EmailMessage
 from email.policy import SMTP
 from email.utils import format_datetime, make_msgid
 
-from ccma.domain.mail_templates import MailTemplate, MailTemplateError, render_mail_template
+from ccma.domain.mail_templates import (
+    MailTemplate,
+    MailTemplateError,
+    render_mail_template,
+    template_spec,
+    validate_block_markers,
+)
 from ccma.domain.models import Member
 from ccma.domain.placeholders import base_placeholder_values
 from ccma.storage.repository import MemberRepository, RepositoryError
@@ -61,10 +67,17 @@ def render_template(
     values: dict[str, str],
     repeats: dict[str, list[dict[str, str]]] | None = None,
 ) -> MailTemplate:
+    template = repository.get_mail_template(key)
     try:
-        return render_mail_template(repository.get_mail_template(key), values, repeats)
+        # The templates are plain files in the store and may have been edited outside
+        # CCMA, so the structural check runs again here: refusing to send beats
+        # sending a mail with "{{#claims}}" in its text.
+        validate_block_markers(key, template.subject, template.body)
+        return render_mail_template(template, values, repeats)
     except MailTemplateError as exc:
-        raise RepositoryError(str(exc)) from exc
+        raise RepositoryError(
+            f"Die E-Mail-Vorlage „{template_spec(key).label}“ ist fehlerhaft: {exc}"
+        ) from exc
 
 
 def compose_mail(