Ask every member to check the data the club stores about them

The club has to keep its member data current, and until now that meant writing
to each member by hand. "Datenüberprüfung anfragen" in the members tab sends one
mail per member, each listing that member's own record: number, name, nickname,
birth date, contact data, address, status, member since, payment frequency, and
the bank details only for members who have any.

Two decisions the record itself forced:

A field with no value is printed as "(nicht hinterlegt)" rather than left out.
The point of the mail is to have gaps filled in, and a missing line is a gap
nobody sees.

The IBAN is masked down to its country code and last four digits. That is enough
to recognise the account, and it keeps a full account number out of a mail the
club sends to dozens of people at once.

Every member is listed as a recipient, with the live memberships that have an
address preselected -- a member who resigned at year's end may still need to
confirm their address, so the board can add them by hand. Members without an
address are skipped and reported instead of failing the run.

Delivery reuses the existing mail machinery: the configured delivery mode, one
SMTP/IMAP connection for the whole run, an archive copy in the member file, a
"data_review_email_sent" event, and the read-only guard before anything is
rendered. A member whose mail fails is reported as a warning and the run
continues -- one bad address must not stop a mailing to the whole club halfway
through. Because the run cannot be taken back, the recipient count is confirmed
once more before it starts.

Subject and text come from a new "Datenüberprüfung" template, editable like the
others, with {{data.sheet}} for the whole record and a {{#data}} block for a
layout of the board's own.

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

tests/test_read_only_store.py

@@ -7,6 +7,7 @@ import pytest
 from ccma.domain.models import ContributionData
 from ccma.services.documents import DocumentService
 from ccma.services.housekeeper import Housekeeper
+from ccma.services.member_data_mail import generate_data_review_mails
 from ccma.services.reminder_mail import generate_and_send_reminder_mail
 from ccma.services.sepa_mail import generate_debit_mails
 from ccma.services.welcome_mail import generate_and_send_welcome_mail
@@ -154,6 +155,16 @@ def test_documents_and_mails_refuse_before_touching_the_member_file(read_only_st
             sender_email="verwaltung@example.org",
             signature="Der Vorstand",
         )
+    with pytest.raises(ReadOnlyStoreError, match="schreibgeschützt"):
+        generate_data_review_mails(
+            repository,
+            [member.member_id],
+            delivery_mode="local",
+            output_directory=tmp_path / "datenpruefung",
+            sender_name="Verwaltung",
+            sender_email="verwaltung@example.org",
+            signature="Der Vorstand",
+        )
 
 
 def test_the_welcome_mail_writes_nothing_at_all_when_refused(read_only_store, tmp_path):