Add GET /payments/:id/receipt — resolves the Stripe-hosted receipt URL
from the payment intent's latest charge (ownership-scoped; returns EFT
receipt directly when present, null otherwise). Wire a "View receipt"
action on completed Stripe rows that fetches the URL on demand and opens
it, with a toast when none is available.
Closes the last billing-audit item (#8).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>