Handle "cooldown" after it's been changed (for server-allocated handles)

[pfrazee]

There's a small issue occurring with folks changing their handle. The old handle gets memorized by folks, but then it gets taken by a new user. (This happened to me -- @paul.bsky.social now gets tagged wrongly all the time 😆 )

My suggestion:

• We introduce a "cooldown" where the handle is reserved for a period after a change
• The cooldown correlates with how long it was used. Maybe something like clamp(used / 2, {min: used/2, max: 1yr}). That way somebody changing their handle rapidly can't jam up the whole system.

In the future we could have a UI warning with mistags of handles on cooldowns, but for now just reserving them would be plenty