Stop double-reporting overdue claims and add SEPA-specific followup

The claim-status and reminder-due housekeeper rules used to run
independently, so once a claim was both overdue and past its dunning
grace period, both a generic "überfällig" task and a "Mahnung fällig"
task showed up for the same problem. Rules can't see each other's
output, so the fix is to give reminder-due sole ownership of the whole
overdue lifecycle: a plain overdue notice during the grace period, then
either the dunning escalation or a fallback overdue notice (dunning
hold active, or waiting between reminder levels) -- never both at once.
claim-status now only handles claims that aren't overdue yet ("bald
fällig").

Members with an active SEPA mandate get a new sepa_debit_overdue finding
instead of the postal dunning escalation once a claim is past its grace
period, since mailing a Mahnung makes no sense for a member who pays by
direct debit -- the board needs to check/retrigger the debit instead.

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

tests/test_housekeeper.py

@@ -29,11 +29,10 @@ def test_housekeeper_reports_initial_payment_and_open_claims(tmp_path) -> None:
     )
 
     findings = Housekeeper(repository).run(today=date(2026, 2, 10))
-    assert {finding.code for finding in findings} == {
-        "initial_payment_overdue",
-        "claim_overdue",
-        "reminder_due",
-    }
+    # Both claims are more than the default 7-day grace period overdue, so the dunning
+    # escalation ("reminder_due") is the single active finding for each -- no separate,
+    # redundant "overdue" finding alongside it.
+    assert {finding.code for finding in findings} == {"reminder_due"}
 
 
 def test_housekeeper_reports_birthdays_before_today_and_after(tmp_path) -> None: