vault: a password change brings a new master key

:passwd used to wrap the same master key under the new password, so an
old diary.db (a pre-* snapshot, an old revision on Proton Drive) and the
old password still opened every sealed file. It now makes a new master
key. The old password opens nothing from that moment; the old master
key is kept, sealed under the new one, in two new vault columns, only
until every sealed file has been re-sealed.

A background worker does that while the vault is open, without holding
it open. Each file gets a new id with its file key wrapped under the new
master key; its records are copied as they are. The move goes through
the same repointing as :encrypt, so the old id is put on backup_retired
and the mirror purges it from Drive. Which files are left is read off
their headers, so a lock or a restart halfway only pauses the work until
the next unlock. Once none is left, the old key is dropped.

Readers take a Keyring that tries the current key, then the retired
one. A second :passwd is refused until the first has finished. :vault
shows the progress. Every sealed file is uploaded to Proton once more.

migrations/0018_vault_rekey.sql

@@ -0,0 +1,7 @@
+-- A changed vault password brings a new master key. Until every sealed file
+-- has been re-sealed under it, the old master key is kept here, sealed under
+-- the new one, so either password-change survivor can still be read; once
+-- the last file is done both columns go back to NULL. The old password opens
+-- neither: it only ever opened the old `sealed`, which is gone.
+ALTER TABLE vault ADD COLUMN retired_nonce BLOB;
+ALTER TABLE vault ADD COLUMN retired_sealed BLOB;