Coalesce duplicate CCH LND invoice trackers

[jjyr]

Summary

• track invoice hashes that are queued or actively monitored by the CCH LND tracker
• coalesce repeated tracking requests for the same invoice
• retry failed trackers only while the invoice is still tracked
• prevent stopped trackers from being requeued after a late failure notification

Motivation

Repeated TrackInvoice messages could enqueue or start multiple LND tracker tasks for the same payment hash. The duplicate work consumed bounded tracker slots and could grow the waiting queue without adding coverage. Keeping a set of queued and active invoice hashes makes tracking idempotent while preserving retries for genuine tracker failures.

Validation

• cargo fmt --all -- --check
• cargo nextest run -p fnn --features sqlite -E 'test(/test_(duplicate_tracking_requests_are_coalesced|stopped_tracker_is_not_requeued_after_failure)/)'
• cargo clippy --all-targets --features sqlite -p fnn -- -D warnings

[gpBlockchain]

lgtm