fix: make Stripe invoice webhooks monotonic

migrations/019_invoice_webhook_invariants.sql

@@ -0,0 +1,16 @@
+DO $$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM invoices
+    WHERE provider_invoice_id IS NOT NULL
+    GROUP BY provider, provider_invoice_id
+    HAVING COUNT(*) > 1
+  ) THEN
+    RAISE EXCEPTION 'duplicate invoices(provider, provider_invoice_id) values require reconciliation before migration 019';
+  END IF;
+END $$;
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_invoices_provider_object_unique
+  ON invoices(provider, provider_invoice_id)
+  WHERE provider_invoice_id IS NOT NULL;