grpc: provision sets the verifier on an already-existing account

An Anope import creates accounts (channel ownership, vhosts, certs) but can't
carry the one-way password hash, so they exist without a verifier. provision
then refused them as AlreadyExists, so the website's backfill could never give
migrated members a credential — every one was locked out of password login.
Make provision update the verifier in place on an existing account (logged as
AccountPasswordSet), and treat an empty scram512 as absent consistently in
apply(). Keyed by name, so imported channel/vhost/ban data stays attached.

src/engine/db/event.rs

@@ -333,7 +333,9 @@ pub(crate) fn apply(accounts: &mut HashMap<String, Account>, channels: &mut Hash
         Event::AccountPasswordSet { account, scram256, scram512 } => {
             if let Some(a) = accounts.get_mut(&key(&account)) {
                 a.scram256 = Some(scram256);
-                a.scram512 = Some(scram512);
+                // Empty = not provided (e.g. a verifier-only backfill sets SHA-256
+                // alone); keep this in step with the live provision update.
+                a.scram512 = (!scram512.is_empty()).then_some(scram512);
             }
         }
         Event::AccountDropped { account } => {