vault: files sealed under a password that is never stored

Argon2id turns the vault password into a key that seals a random master
key; only the sealed copy is kept. Each file gets its own key, sealed
under the master key in its header with its media id, and its bytes are
AES-256-GCM STREAM records, so a swapped, cut or altered file refuses
to open. Uploads with ?encrypt=true are sealed as they stream in.

:vault, :unlock, :lock, :passwd, :encrypt, :decrypt and :upload! drive
it. The vault locks after WORKSPACE_VAULT_IDLE_MIN idle minutes, on
logout and on restart; while locked, sealed files answer 423, share
links never serve them, and :export! refuses rather than leave them out.

Sealing an existing file rewrites it under a new id and retires the old
one: the mirror deletes the plaintext from Proton Drive outright,
trash included, whatever WORKSPACE_BACKUP_PRUNE says, and drops it from
the readable files/ view. The mirror now compares on-disk sizes, so
sealed files are not re-hashed every run.

.env.example

@@ -90,3 +90,7 @@ WORKSPACE_DIGEST_AT=sun 18:00
 
 # Which of the four kinds may be sent. Drop one from the list to switch it off.
 WORKSPACE_MAIL_KINDS=reminder,card_due,digest,login
+
+# Minutes the vault stays open with nothing using it, before it locks itself.
+# It also locks on logout and whenever the server restarts.
+WORKSPACE_VAULT_IDLE_MIN=15