Send each scheduled payment's due notice exactly once

The daily billing scan runs on request via WP-Cron and can overlap
itself under concurrent traffic. Each run emailed the payments it
created with no record that a notice had gone out, so two overlapping
runs could send a payer two identical "Payment due" emails for one
charge — read by families as being billed twice, though only one row
exists.

Stamp us_payments.notice_sent_at atomically before emailing: the scan
now claims each payment with a conditional UPDATE ... WHERE
notice_sent_at IS NULL and only notices, credits and batches the rows
it won. A competing run finds them claimed and stays quiet, so exactly
one notice is sent regardless of how the scan is triggered. A one-time
backfill stamps existing scheduled rows on upgrade so already-noticed
charges are not re-emailed.

Co-authored-by: anthropic/claude-opus-4-8

src/Payment/PaymentService.php

@@ -106,6 +106,16 @@ class PaymentService {
 		$this->payments->assignNoticeBatch( $ids, $batch );
 	}
 
+	/**
+	 * Atomically claim a payment for its single due-payment notice, returning
+	 * whether this call won the claim. The daily scan calls this before emailing
+	 * so a payment is noticed exactly once even when WP-Cron overlaps. Delegates
+	 * to the ledger.
+	 */
+	public function markNoticed( int $paymentId ): bool {
+		return $this->payments->markNoticed( $paymentId );
+	}
+
 	/**
 	 * Studio-admin confirmation that a pending payment (e-transfer) was received.
 	 * Marks it paid, confirms the registration, and emails the receipt.