lnurl: budget unauthenticated invoice generation per recipient

GET /pay/:user/:amount and the lnurl callback create a CLN invoice and an
entry in the recipient's invoice list for anyone holding the link, which a
scanner on rotating IPs can do without tripping per-IP limits. Cap fresh
invoices at 120 per recipient per hour (reused invoices don't count).

Also fail cleanly for unknown usernames and skip invoice ids whose records
no longer exist, instead of throwing on the null and breaking the pay page.
Throttle the resulting lnurlp warnings per username.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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