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.

web/src/lib/api.ts

@@ -199,6 +199,11 @@ export interface VaultStatus {
   locks_in: number | null
   /** How many files are sealed: what an export needs the vault open for. */
   sealed: number
+  /**
+   * Present while files sealed before the last password change are still
+   * being re-sealed under the new key; `total` is 0 until counted.
+   */
+  rekey: { done: number; total: number } | null
 }
 
 /** The Proton Drive mirror, as `/api/backup` reports it. */