feat(payments): include invoice number in overdue payment reference

[lago-claude-ai-agent]

When Lago auto-charges an overdue payment request off-session, the reference sent to the payment provider was hardcoded to <billing entity> - Overdue invoices with no invoice number, so reconciling a payment back to an invoice from a provider report meant looking it up in Lago. The checkout-link path (PaymentRequests::Payments::StripeService#description) already appends the number when there is a single invoice; the auto-charge path now does the same, so both paths send the same reference.

Payment requests covering several invoices are deliberately unchanged: listing every number needs a per-provider truncation budget (Adyen caps reference at 80 chars) that has no precedent in the codebase.

[lago-claude-ai-agent]

PASS — the auto-charge path now builds the same reference as PaymentRequests::Payments::StripeService#description, the logic is copied verbatim from that proven pattern, and the new spec pins it.

Verified: payment_reference matches the existing Invoices::Payments::CreateService#payment_reference shape and placement; the single-invoice spec relies on the with(reference:) stub constraint, so it would not pass on the old string; all reference consumers just forward the value; 2 files, no unrelated churn.

Non-blocking notes for the human reviewer:

• The multi-invoice case is deliberately left unchanged. That keeps the two Stripe paths consistent and is disclosed in the description, but it is a product call worth an explicit ack.
• PaymentRequests::Payments::Adyen Service#payment_url_params still sends reference: "Overdue invoices" — no billing entity, no invoice number. Pre-existing divergence, not introduced here, but it means the new code comment's "both payment paths" only holds for Stripe.
• reference reaches Adyen's payment_params, where the field caps at 80 chars; a long billing entity name plus an invoice number gets closer to that. Same exposure the invoice auto-charge path already has, so no new class of problem.