add DeleteForwardingHistory to selectively purge old forwarding history

Routing nodes and LSPs accumulate forwarding history over time which, if
compromised or subpoenaed, could reveal sensitive information about payment
flows. Prior to this change, the only way to purge old forwarding history was
to reset the database and restore from backup — a disruptive operation with
significant operational risk.

This PR adds a DeleteForwardingHistory RPC to the Router sub-server that
allows operators to permanently delete forwarding events older than a specified
time threshold, without any node downtime.

https://github.com/lightningnetwork/lnd/pull/10666