Stop rolling back reminders/SEPA mails after a successful send

The previous fix reverted the reminder to "draft" on any failure after
mark_reminder_sent, including failures that happened after the mail had
already been handed to send_via_smtp or appended to an IMAP folder. That
made a successful send followed by a Sent-folder-copy or archive-move
failure look like nothing was sent, inviting a duplicate send/fee booking
on retry -- the same "sent" flag needs to be preserved once delivery is
no longer reversible, per follow-up review.

reminder_mail.generate_and_send_reminder_mail and
sepa_mail.generate_debit_mails now track whether the mail actually left
the building (SMTP accepted / IMAP append succeeded / local file written)
separately from the later archiving step:
- Failure before that point: reminder_mail reverts to draft (unchanged);
  sepa_mail now records a warning and continues with the remaining
  debits instead of aborting the whole batch.
- Failure after that point (Sent-copy append, moving the archive file
  into place): the reminder stays "sent" / the debit stays in the
  batch's results, an event is still logged for traceability (with an
  archive_error note and no document reference), and the caller gets a
  clear error to follow up on manually -- no rollback, no silent loss of
  the fact that the mail already went out.

tests/test_reminder_mail.py

@@ -301,3 +301,72 @@ def test_reminder_mail_local_write_failure_reverts_sent_status(tmp_path, monkeyp
 
     data = repository.get_contributions(member.member_id)
     assert data.reminders[0]["status"] == "draft"
+
+
+def test_reminder_mail_archiving_failure_after_smtp_success_does_not_revert(tmp_path, monkeypatch):
+    import ccma.services.reminder_mail as reminder_mail_module
+
+    repository, member = _overdue_claim_repository(tmp_path / "store")
+    member.email = "reminder@example.org"
+    repository.save_member(member)
+    repository.save_email_settings(
+        delivery_mode="send",
+        smtp_host="mail.example.org",
+        smtp_port=587,
+        smtp_security="none",
+        smtp_username="",
+        smtp_password="",
+        imap_host="",
+        imap_port=993,
+        imap_security="ssl",
+        imap_username="",
+        imap_password="",
+        imap_drafts_folder="",
+    )
+    reminder = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Zahlungserinnerung",
+        payment_deadline_days=14,
+        items=[{"description": "Mahngebühr", "amount": "5.00"}],
+    )
+
+    @contextmanager
+    def _fake_smtp_session(settings):
+        yield "smtp-client"
+
+    monkeypatch.setattr(reminder_mail_module, "smtp_session", _fake_smtp_session)
+    monkeypatch.setattr(reminder_mail_module, "send_via_smtp", lambda client, content: None)
+
+    real_replace = reminder_mail_module.os.replace
+
+    def _flaky_replace(src, dst):
+        # Only the final archive move (into the "Mahnungen" folder) should fail --
+        # the mail has already been handed to send_via_smtp by that point.
+        if str(dst).endswith(".eml") and "Mahnungen" in str(dst):
+            raise OSError("disk full")
+        return real_replace(src, dst)
+
+    monkeypatch.setattr(reminder_mail_module.os, "replace", _flaky_replace)
+
+    with pytest.raises(RepositoryError, match="zugestellt"):
+        generate_and_send_reminder_mail(
+            repository,
+            member.member_id,
+            "claim-1",
+            reminder["reminder_id"],
+            delivery_mode="send",
+            sender_name="Verwaltung C3MA",
+            sender_email="verwaltung@example.org",
+            signature="Der Vorstand",
+        )
+
+    data = repository.get_contributions(member.member_id)
+    stored_reminder = data.reminders[0]
+    assert stored_reminder["status"] == "sent", "mail was already sent -- must not be un-sent"
+    assert stored_reminder.get("fee_item_ids")
+    events = repository.get_events(member.member_id)
+    assert events[-1].event_type == "reminder_email_sent"
+    assert events[-1].data["archive_error"]
+    assert "document" not in events[-1].references