fix: invoice integrity — atomic numbering, safe delete, single create path

- Invoice numbers now come from DB function next_invoice_number()
  (max+1 per year under advisory lock) with a unique index; the old
  row-count method reused numbers after deletes and raced concurrent
  creates, which broke public pay links
- Remove dead standalone TeamCreateInvoice page; the TeamInvoices
  modal is the single create path (page had already drifted)
- Invoice delete now asks for confirmation and only un-bills tasks/
  submissions not billed on another invoice
- Created invoice shows correct "sent" status in list without reload
- Invoice/due dates computed at save time, not module load
- Reopening a paid invoice clears stale stripe_fee
- Stripe webhook markPaid is idempotent (no double receipts)
- subcontractor_invoice_items.version_number column stores billing
  version explicitly; description parsing kept as legacy fallback
- Drop unused buildInvoiceStatusByKey/deriveVersionStatus

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/pages/external/ExternalMyInvoices.jsx

@@ -55,7 +55,7 @@ function asArray(value) {
 }
 
 function parseItemVersionNumber(item) {
-  if (Number.isFinite(Number(item?.version_number))) return Number(item.version_number);
+  if (item?.version_number != null && Number.isFinite(Number(item.version_number))) return Number(item.version_number);
   const match = String(item?.description || '').match(/\bR(\d{2})\b/i);
   return match ? Number(match[1]) : 0;
 }