Persists per-username failed-attempt counts and computed locked_until
timestamps. Lockout service computes exponential-backoff durations
(min(base * 2^(count-threshold), max)) with auto-unlock once locked_until
passes. Successful login deletes the row, resetting the counter.
Pure DB-keyed lockout — survives server restarts and shares state across
both login routes (HTML and JSON) when wired in a later step.
