Handle failed direct debits through the same reminder mechanism

A Rücklastschrift is still the member's own failure to ensure cover or
provide valid bank details, so it belongs in the same escalation ladder
as a regular reminder -- it now occupies whatever level the claim is next
due for, alongside a "Rücklastschrift" preset next to the existing
Mahnstufe presets.

Mahnung vorbereiten no longer lets the board pick a level (it's computed
automatically from what's already been sent, since the sequencing was
already server-enforced) and replaces the single fee field with a small
items table (Beschreibung + Betrag), so a reminder can carry several
charges at once -- e.g. Rücklastschriftgebühr plus Bankgebühr plus Porto
-- each landing on the claim as its own line item instead of one lump
sum under a generic label. The description field offers the configured
standard texts but stays free-editable.

Those standard texts and amounts, plus the dunning levels themselves
(name/fee/deadline), are now configurable from Optionen -> Mahnungen
instead of only being editable by hand-editing repository.json, which
was the case for the whole reminder policy until now. Cleaned up
reminder_fee/failed_debit_fee on contribution rules while at it -- both
were unused leftovers superseded by this.

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

tests/test_reminder_mail.py

@@ -64,7 +64,7 @@ def test_reminder_mail_includes_fee_in_open_balance(tmp_path):
         level=1,
         name="Erste Mahnung",
         payment_deadline_days=14,
-        fee="5.00",
+        items=[{"description": "Mahngebühr", "amount": "5.00"}],
     )
 
     generated = generate_and_send_reminder_mail(