Remove the SSHA512 password hashes

All passwords are migrated to bcrypt, so {SSHA512} hashes are no longer
checked; an unknown scheme raises like before. The README documents that
Dovecot must support BLF-CRYPT.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -24,6 +24,11 @@ value the client sends. Max uses this address for audit logging. For nginx:
 proxy_set_header X-Real-IP $remote_addr;
 ```
 
+## Dovecot
+
+Dovecot reads the password hashes written by Max from the `users` table. Max stores bcrypt
+hashes as `{BLF-CRYPT}$2b$12$…`. Dovecot must therefore support `BLF-CRYPT` (check with `doveadm pw -l`).
+
 ## Dependencies
 
 Direct dependencies are listed in `requirements.in` (production) and