finalize: don't crash on non-integer fee delta (db.incrBy NaN)

Triggered by a recurring "finalize threw despite no failed_parts: ERR
value is not an integer or out of range" warning. Root cause: when xpay's
response shape is unexpected (or invoice is zero-amount and decoded
amount_msat is undefined), the computed p.fee becomes NaN. The
subsequent db.incrBy(balance:<uid>, maxfee - NaN) throws because Redis
can't increment by NaN.

Symptom: leak guard fires "ref set in db despite throw" — payment is
actually fine (p.ref persisted before the throw) but finalize never
completes the fee refund. User loses up to maxfee sats (was already
debited maxfee, gets nothing back).

Fix: validate the computed fee is a finite integer; if not, keep the
originally-reserved maxfee (zero refund, no balance corruption) and warn
loudly. Same guard on the refund delta before db.incrBy.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

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