backup: keep Proton key passphrases, never the password

The stored session used to hold the mailbox password, which on a
single-password account is the login password. It now holds the per-key
passphrases derived from it at login, which unlock the keys but cannot
log in, change the password or turn 2FA off. Login also checks them
against the account's keys, so a wrong mailbox password fails there
rather than at the first backup.

A session stored by an older build is upgraded on the next start: its
key salts are already known, so the passphrases derive locally and the
file is rewritten without the password.

README.md

@@ -257,10 +257,14 @@ narl-workspace proton-login                       # or, in Docker:
 docker compose exec -it workspace narl-workspace proton-login
 ```
 
-That stores a session — tokens, the mailbox password needed to rebuild the key
-chain, and the account key salts — as a `0600` file next to the database. It
-sits on the same volume as the workspace it protects, and it is enough to read the
-account, so the volume is the thing to keep private. Built with
+That checks the password against the account's keys and stores a session —
+the tokens, and the key passphrases derived from the password at login — as a
+`0600` file next to the database. The password itself is never stored: the
+passphrases unlock the keys but cannot log in, change the password or turn 2FA
+off. They are still enough to read the account, and the file sits on the same
+volume as the workspace it protects, so the volume is the thing to keep private.
+A session stored by an older build, which kept the password, is rewritten
+without it on the next start; no new login is needed. Built with
 `--features keyring` on a host that has a Secret Service, the session goes to
 the OS keyring instead and the file is only a fallback; the container has no
 session bus, so there it is always the file.