feat(logs): keep the username on records the account no longer answers for

The connection log took the name from a join on `users`, and deletion
tombstones that row — so every record belonging to a deleted account
reported `deleted-3f9a1c`, which is the one answer that helps nobody. The
log is kept for a legal retention period precisely so it can say who did
what; losing the name at deletion kept the data and lost the point of it.

`ip_logs.username` is written as the account is erased, and stays NULL
while the account is alive, where the join is better because it cannot go
stale. The admin view prefers the stored name when there is one: the join
still answers after deletion, just with the tombstone.

Releasing the username for re-registration and keeping it in the log are
separate things, and the guide now says so.

On the node side, the pre-proof audit line records the username the
session already knew, instead of leaving the column empty.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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