refactor: batch paykit contact cleanup

[ben-kaufman]

Description

This PR builds on #637 to address the Paykit cleanup performance follow-ups from #620:

• Reads the SDK linked-peer list once per multi-contact publication or cleanup pass instead of once per contact.
• Clears receiver-scoped payment lists as before, then drains outbound private messages and checks pending status once for the whole successful batch.
• Preserves per-contact failure isolation: contacts with failed or still-pending cleanup retain their cached state and retry marker, while successful contacts are cleared.

No user-facing behavior changes are intended.

Linked Issues/Tasks

• Base PR: feat: support incoming paykit requests #637
• Review follow-up: feat: support paykit receiver paths #620
• Companion Android PR: refactor: batch paykit contact cleanup bitkit-android#1099

Screenshot / Video

N/A — service-only refactor.

QA Notes

Manual Tests

N/A

Automated Checks

• PrivatePaykitServiceTests.swift: all 10 focused tests passed in the iOS simulator.
• SwiftFormat lint passed for the changed Swift file.
• git diff --check passed.

[greptile-apps]

Greptile Summary

This PR batches Paykit contact cleanup work to reduce repeated SDK calls. The main changes are:

• Reads and groups linked peers once per publication or cleanup batch.
• Drains outbound private messages once after clearing the batch.
• Tracks cleanup success and cached-state removal per contact.

Confidence Score: 4/5

The batch-wide linked-peer failure path can leave stale endpoints without a cleanup retry.

• Successful batch processing keeps failures isolated by contact.
• A failed shared lookup suppresses cleanup updates and retry keys for every contact.
• Non-immediate publication callers can discard that failure.

Bitkit/Services/PrivatePaykitService+Contacts.swift

Important Files Changed

Filename	Overview
Bitkit/Services/PrivatePaykitService+Contacts.swift	Batches linked-peer reads and message draining while preserving per-contact cleanup state, but the shared lookup failure path can omit cleanup and retries.

_{Reviews (1): Last reviewed commit: "refactor: batch paykit contact cleanup" | Re-trigger Greptile}