invoices: don't let a supplied id overwrite another user's invoice

generate() writes invoice:<id> wholesale with the caller's uid, and the old
bolt11 pointer keeps resolving to that id, so passing someone else's invoice
id re-pointed their invoice at the caller's account. Invoice ids aren't
secret (LUD-21 verify URLs, invoice pages, receipts), so refuse an existing
id that belongs to a different user, both in generate() and in POST /invoice
for anonymous pay-page callers. Both fail with "invoice not found" so ids
can't be probed for existence.

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

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