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>

supabase/functions/stripe-webhook/index.ts

@@ -25,6 +25,16 @@ serve(async (req) => {
 
   // Helper: retrieve fee from a payment intent and mark invoice paid
   async function markPaid(paymentIntentId: string, invoice_id: string) {
+    // Idempotency: Stripe retries events, and card payments fire both
+    // checkout.session.completed and payment_intent.succeeded. Never
+    // overwrite paid_at or re-send the receipt for an already-paid invoice.
+    const { data: existing } = await supabase
+      .from('invoices')
+      .select('status')
+      .eq('id', invoice_id)
+      .single();
+    if (existing?.status === 'paid') return;
+
     let stripe_fee: number | null = null;
     try {
       const pi = await stripe.paymentIntents.retrieve(paymentIntentId, {