fix(documents): anchor floating supplier-invoice underlag instead of nagging (#1248)

A verifikat booked from a supplier invoice showed the invoice PDF when opened
while the list kept warning "Underlag saknas" on the same row. Both surfaces
behaved as written: every missing-underlag surface only accepts a referenced
supplier-invoice document when it is ANCHORED to a journal entry (only anchored
docs sit behind block_document_deletion), while the verifikat view's reference
resolver displayed the document regardless.

The document was floating because delete_last_voucher clears journal_entry_id
on everything attached to the voucher it tears down (the FK is ON DELETE
RESTRICT, so it must). Deleting a rättelse the invoice PDF had been relinked
onto therefore orphaned it while the payment verifikat stayed posted, and
nothing ever anchored it again: the warning was unresolvable by design.

Same class one surface over: v1 mark-paid never linked the document at all,
dashboard mark-paid only did so for the cash entry, and both
match-supplier-invoice routes propagated the transaction's document but not the
invoice's own. Four of the five affected prod rows come from those paths, not
from a deleted voucher.

- lib/core/documents/supplier-invoice-underlag.ts: anchor a floating document
  to the invoice's own posted verifikat (registration, then payment, then
  partial payments; open unlocked periods only). Never moves an anchored doc,
  never throws.
- Called after delete_last_voucher and from all four payment paths.
- getJournalEntryUnderlagReferences withholds an unanchored document so the
  verifikat view and the warning can no longer contradict each other.
- Migration 20260727180000 backfills the rows already in this state.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/api/transactions/[id]/match-supplier-invoice/route.ts

@@ -9,6 +9,7 @@ import { cancelOrphanedPaymentEntry } from '@/lib/bookkeeping/cancel-orphaned-en
 import { planSupplierPayment } from '@/lib/invoices/apply-supplier-payment'
 import { createJournalEntry, findFiscalPeriod } from '@/lib/bookkeeping/engine'
 import { isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { anchorSupplierInvoiceDocument } from '@/lib/core/documents/supplier-invoice-underlag'
 import { withRouteContext } from '@/lib/api/with-route-context'
 import { errorResponse, errorResponseFromCode } from '@/lib/errors/get-structured-error'
 import { validateBody } from '@/lib/api/validate'
@@ -451,6 +452,13 @@ export const POST = withRouteContext(
       }
     }
 
+    // Same requirement one table over: the SUPPLIER INVOICE's own retained
+    // document must sit on a posted verifikat, or the payment verifikat we
+    // just booked shows the invoice PDF while every missing-underlag surface
+    // (which only accepts an anchored doc) warns "Underlag saknas". No-op when
+    // it is already anchored, e.g. on the registration verifikat.
+    await anchorSupplierInvoiceDocument(supabase, companyId, supplier_invoice_id)
+
     logMatchEvent(supabase, user.id, transactionId, 'matched', {
       supplierInvoiceId: supplier_invoice_id,
       matchConfidence: 1.0,