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were unused leftovers superseded by this.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>

src/ccma/domain/contributions.py

@@ -153,6 +153,13 @@ def donation_status(data: ContributionData, donation: dict[str, Any]) -> str:
     return "open"
 
 
+def reminder_items_total(reminder: dict[str, Any]) -> Decimal:
+    return sum(
+        (decimal_value(item.get("amount", "0")) for item in reminder.get("items") or []),
+        Decimal("0"),
+    )
+
+
 def contribution_override_covers_month(override: dict[str, Any], month: str) -> bool:
     valid_from = str(override.get("valid_from", ""))
     valid_until = str(override.get("valid_until", "") or "")