payments: guard tip credit and self-referential autowithdraw

Clamp a non-internal receipt's tip to the settled amount so a tip
mutated larger than the amount actually received can't produce a
negative, reversible payment record on the recipient.

Also reject an autowithdraw whose destination resolves back to the
sender: that self-send re-fires completePayment and, since the balance
nets out, never drops below threshold, looping unbounded. The route's
create() already has the equivalent self-send guard; the internal
pay() helper lacked it.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Dpqu6wizYe1c1g4k5Gd8JK

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