preimages: enforce a merchant-set price per preimage batch

Invoice creation is unauthenticated, so buyers could name their own
price for a queued preimage (create the usePreimage invoice with
amount: 1). POST /preimages now takes an optional price (sats) that is
stored per entry and overrides the requested amount when the preimage
is consumed; omitting it keeps the old any-amount behavior. Also gate
the stored-preimage reveal in credit() on the invoice being fully paid,
since internal sends can settle partial amounts. Bare-hex entries
queued before this change still parse (treated as priceless).

Co-Authored-By: Claude Fable 5 <[email protected]>

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