fix(payment): accept paid quote issuers within K closest peers

[mickvandijke]

Summary

• Widen single-node paid quote issuer locality from the storage close-group width to the Kademlia K width (K_BUCKET_SIZE).
• Keep direct client PUT receiver storage admission unchanged at close group plus storage margin.
• Update verifier docs/test helpers and add a unit test pinning paid quote issuer locality to K.

SemVer

• Patch: payment verifier admission policy fix.

Verification

• cargo test issuer
• cargo test payment::verifier::tests
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings