Allow editing reminders (incl. undoing "sent") and prefill a Rücklastschrift reason

A generated/sent Mahnung email couldn't be corrected before: sent reminders were
locked, so a wording mistake meant living with it. Add edit_reminder_draft() for
draft/generated reminders and revert_reminder_sent() to safely undo a "sent"
reminder (removing the fee items it booked, refusing if a payment already covers
them or a higher Mahnstufe already exists) so it can be edited and resent.

ReminderDialog now supports an edit mode (reminder=... prefills name/detail/
items/deadline/channel, level stays fixed) and claim_tab gained a "Mahnung
bearbeiten" button that reverts-then-edits for sent reminders automatically,
asking for confirmation first since it un-books the fee.

The Rücklastschrift preset now also prefills "Details" with a short explanation
("Leider konnte die Lastschrift ... nicht eingelöst werden.") since that field
goes straight into the reminder email -- the board can append the concrete
reason (mangels Deckung, Konto ungültig, ...) right there instead of the field
starting empty.

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

tests/test_reminders.py

@@ -281,6 +281,142 @@ def test_reminder_policy_rejects_invalid_input(tmp_path) -> None:
         )
 
 
+def test_edit_reminder_draft_updates_fields_but_not_level(tmp_path) -> None:
+    repository, member = _overdue_claim_repository(tmp_path)
+    draft = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Zahlungserinnerung",
+        payment_deadline_days=14,
+        items=[],
+    )
+
+    edited = repository.edit_reminder_draft(
+        member.member_id,
+        "claim-1",
+        draft["reminder_id"],
+        name="Erste Mahnung",
+        payment_deadline_days=10,
+        detail="Bitte um Ausgleich.",
+        items=[{"description": "Mahngebühr", "amount": "5.00"}],
+    )
+    assert edited["level"] == 1
+    assert edited["name"] == "Erste Mahnung"
+    assert edited["payment_deadline_days"] == 10
+    assert edited["detail"] == "Bitte um Ausgleich."
+    assert edited["items"] == [{"description": "Mahngebühr", "amount": "5.00"}]
+
+
+def test_edit_reminder_draft_rejects_already_sent_reminder(tmp_path) -> None:
+    repository, member = _overdue_claim_repository(tmp_path)
+    draft = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Zahlungserinnerung",
+        payment_deadline_days=14,
+        items=[],
+    )
+    repository.mark_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+
+    with pytest.raises(RepositoryError, match="Nur ein Entwurf"):
+        repository.edit_reminder_draft(
+            member.member_id,
+            "claim-1",
+            draft["reminder_id"],
+            name="Andere Bezeichnung",
+            payment_deadline_days=14,
+        )
+
+
+def test_revert_reminder_sent_removes_booked_fee_and_reopens_draft(tmp_path) -> None:
+    repository, member = _overdue_claim_repository(tmp_path)
+    draft = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Rücklastschrift",
+        payment_deadline_days=14,
+        items=[{"description": "Rücklastschriftgebühr", "amount": "5.00"}],
+    )
+    sent = repository.mark_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+    assert len(sent["fee_item_ids"]) == 1
+    _data, claim = repository.get_claim(member.member_id, "claim-1")
+    assert claim_total(claim) == Decimal("105.00")
+
+    reverted = repository.revert_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+    assert reverted["status"] == "draft"
+    assert reverted["sent_at"] is None
+    assert reverted["payment_deadline"] is None
+    assert reverted["fee_item_ids"] == []
+
+    _data, claim = repository.get_claim(member.member_id, "claim-1")
+    assert claim_total(claim) == Decimal("100.00")
+    descriptions = {item["description"] for item in claim["items"]}
+    assert "Rücklastschriftgebühr" not in descriptions
+
+    # Now editable and re-sendable, e.g. after fixing the wording.
+    repository.edit_reminder_draft(
+        member.member_id,
+        "claim-1",
+        draft["reminder_id"],
+        name="Rücklastschrift",
+        payment_deadline_days=14,
+        items=[{"description": "Rücklastschriftgebühr", "amount": "7.50"}],
+    )
+    repository.mark_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+    _data, claim = repository.get_claim(member.member_id, "claim-1")
+    assert claim_total(claim) == Decimal("107.50")
+
+
+def test_revert_reminder_sent_rejects_when_higher_level_exists(tmp_path) -> None:
+    repository, member = _overdue_claim_repository(tmp_path)
+    first = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Zahlungserinnerung",
+        payment_deadline_days=14,
+        items=[],
+    )
+    repository.mark_reminder_sent(member.member_id, "claim-1", first["reminder_id"])
+    repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=2,
+        name="Erste Mahnung",
+        payment_deadline_days=14,
+        items=[{"description": "Mahngebühr", "amount": "5.00"}],
+    )
+
+    with pytest.raises(RepositoryError, match="höhere Mahnstufe"):
+        repository.revert_reminder_sent(member.member_id, "claim-1", first["reminder_id"])
+
+
+def test_revert_reminder_sent_rejects_when_payment_already_covers_fee(tmp_path) -> None:
+    repository, member = _overdue_claim_repository(tmp_path)
+    draft = repository.create_reminder_draft(
+        member.member_id,
+        "claim-1",
+        level=1,
+        name="Rücklastschrift",
+        payment_deadline_days=14,
+        items=[{"description": "Rücklastschriftgebühr", "amount": "5.00"}],
+    )
+    repository.mark_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+    repository.record_payment(
+        member.member_id,
+        "claim-1",
+        payment_date="2026-02-10",
+        amount="105.00",
+        allocation_amount="105.00",
+    )
+
+    with pytest.raises(RepositoryError, match="bereits Zahlungen verbucht"):
+        repository.revert_reminder_sent(member.member_id, "claim-1", draft["reminder_id"])
+
+
 def test_reminder_policy_backfills_standard_fee_items_for_old_repositories(tmp_path) -> None:
     repository = MemberRepository(tmp_path)
     repository.initialize()