SC-01: unlock_event now verifies caller has access to the calendar before
revealing lock state. Previously any authenticated user could probe event
existence via 404/204/403 response differences.
SC-02: acquire_lock no longer overwrites permanent locks. If the owner holds
a permanent lock and clicks Edit, the existing lock is returned as-is instead
of being downgraded to a 5-minute temporary lock.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>