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>

src/ccma/storage/repository.py

@@ -17,6 +17,8 @@ from ccma.domain.contributions import (
     CONTRIBUTION_OVERRIDE_KIND_LABELS,
     allocated_total,
     claim_balance,
+    claim_items,
+    claim_settled_total,
     claim_total,
     contribution_override_ranges_overlap,
     credit_allocated_total,
@@ -2108,6 +2110,97 @@ class MemberRepository:
             references={"claim_id": claim_id, "reminder_id": reminder_id},
         )
 
+    def edit_reminder_draft(
+        self,
+        member_id: str,
+        claim_id: str,
+        reminder_id: str,
+        *,
+        name: str,
+        payment_deadline_days: int,
+        detail: str = "",
+        items: list[dict[str, str]] | None = None,
+        channel: str = "email",
+    ) -> dict:
+        try:
+            normalized_items = _normalize_reminder_items(items)
+        except ValueError as exc:
+            raise RepositoryError(str(exc)) from exc
+        if payment_deadline_days < 1 or payment_deadline_days > 365:
+            raise RepositoryError("Die Zahlungsfrist muss zwischen 1 und 365 Tagen liegen.")
+        data, _claim = self.get_claim(member_id, claim_id)
+        reminder = self._find_reminder(data, claim_id, reminder_id)
+        if str(reminder.get("status", "draft")) not in {"draft", "generated"}:
+            raise RepositoryError("Nur ein Entwurf kann bearbeitet werden.")
+        reminder["name"] = name.strip() or reminder["name"]
+        reminder["detail"] = detail.strip()
+        reminder["channel"] = channel.strip() or "email"
+        reminder["payment_deadline_days"] = payment_deadline_days
+        reminder["items"] = normalized_items
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="reminder_draft_edited",
+            summary=f"Mahnungsentwurf bearbeitet: {reminder['name']}",
+            references={"claim_id": claim_id, "reminder_id": str(reminder["reminder_id"])},
+            data={"fee": money_text(reminder_items_total(reminder))},
+        )
+        return reminder
+
+    def revert_reminder_sent(self, member_id: str, claim_id: str, reminder_id: str) -> dict:
+        """Undoes mark_reminder_sent: removes the fee items it booked onto the claim
+        and puts the reminder back into "draft" so its text/items can be edited and it
+        can be sent again -- e.g. after generating a Mahnung email and noticing the
+        wording needs a fix. Refuses if a later Mahnstufe already exists (would break
+        the level ordering) or if payments already booked against the claim would no
+        longer fit once the fee is removed."""
+        data, claim = self.get_claim(member_id, claim_id)
+        reminder = self._find_reminder(data, claim_id, reminder_id)
+        if str(reminder.get("status", "")) != "sent":
+            raise RepositoryError("Nur eine bereits versandte Mahnung kann zurückgesetzt werden.")
+        if any(
+            str(item.get("claim_id", "")) == claim_id
+            and int(item.get("level", 0)) > int(reminder.get("level", 0))
+            and str(item.get("status", "draft")) in {"draft", "generated", "sent"}
+            for item in data.reminders
+        ):
+            raise RepositoryError(
+                "Es existiert bereits eine höhere Mahnstufe; diese Mahnung kann nicht "
+                "zurückgesetzt werden."
+            )
+        fee_item_ids = set(reminder.get("fee_item_ids") or [])
+        if fee_item_ids:
+            removed_total = sum(
+                (
+                    decimal_value(item.get("amount", "0"))
+                    for item in claim_items(claim)
+                    if str(item.get("item_id", "")) in fee_item_ids
+                ),
+                Decimal("0"),
+            )
+            new_total = claim_total(claim) - removed_total
+            if claim_settled_total(data, claim) > new_total:
+                raise RepositoryError(
+                    "Für diese Forderung wurden bereits Zahlungen verbucht, die die "
+                    "Mahngebühr abdecken; der Versand kann nicht zurückgesetzt werden."
+                )
+            claim["items"] = [
+                item for item in claim_items(claim) if str(item.get("item_id", "")) not in fee_item_ids
+            ]
+            claim["amount"] = money_text(claim_total(claim))
+        reminder["status"] = "draft"
+        reminder["sent_at"] = None
+        reminder["payment_deadline"] = None
+        reminder["fee_item_ids"] = []
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="reminder_sent_reverted",
+            summary=f"Versand zurückgesetzt: {reminder.get('name', '')}",
+            references={"claim_id": claim_id, "reminder_id": str(reminder["reminder_id"])},
+        )
+        return reminder
+
     def register_reminder_document(
         self,
         member_id: str,