Fix invoice.upcoming webhook handler failure - generate unique document ID for invoices without id#706

Copilot wants to merge 2 commits intonextinvertase/stripe-firebase-extensions:nextfrom

copilot/fix-1b851cc4-07ae-4035-8141-06854041e216invertase/stripe-firebase-extensions:copilot/fix-1b851cc4-07ae-4035-8141-06854041e216Copy head branch name to clipboard