V216: how deep a storage profile's keys are fanned out.

phoenix_kit_storage_profiles.key_levels (smallint, NOT NULL, default 1) is the number of two-character hash folders between a library's key prefix and a file's own folder:

0   <prefix>/<md5>/…
1   <prefix>/22/<md5>/…           (every key so far)
2   <prefix>/22/e8/<md5>/…
3   <prefix>/22/e8/b9/<md5>/…

A deeper layout keeps any one directory short in a library of many millions of files on a local disk (PhoenixKit.Modules.Storage.KeyLayout has the numbers the Storage profiles tab shows). It applies to new uploads into the profile's libraries: a file's folder is stored on its row, so every file already uploaded stays where it is and is found as before.

Every existing profile gets 1, which is the layout in use. The column is not part of the profile's placement, so changing it does not bump the profile's revision and no file becomes out of date.

Locks

ADD COLUMN … NOT NULL DEFAULT 1: a constant default, so Postgres adds it without rewriting phoenix_kit_storage_profiles (a handful of rows in any case). Re-runnable.