Maity: Board

hub: reserve the lt-<n> handle prefix for load-test accounts

tsk-ch8nt5dfgmp7 open fix

Reserve the handle prefix lt-<digits> for load-test accounts, so that no real person can sign up as lt-7.

Why: the load-test seed (#1186) names its accounts lt-<n>, and maity-hub loadtest purge (#1188) deletes them. #1188 additionally checks the seed marker in password_hash; this card closes the remaining gap: a real account with such a handle is confusing in the test and should not exist.

Scope: lt-[0-9]+ in paths.handle_reserved and the §16.11 lists in docs/api.md in the same PR (contract and hub tests change together), a hub test for sign-up and rename, KTW note in hub/context/replicas.md. Existing accounts with such a handle: check first, not renamed automatically.

Found by c47w in the review of #1186/#1188 (seq 36750, 36760).