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/account.rs

@@ -42,12 +42,28 @@ impl Db {
     /// can't clobber a fully-credentialed account). `scram512` empty =
     /// SCRAM-SHA-512 unavailable for this account (it falls back to 256).
     pub fn provision_account(&mut self, name: &str, scram256: &str, scram512: &str, email: Option<String>) -> Result<(), RegError> {
-        if self.exists(name) {
-            return Err(RegError::Exists);
-        }
         if name.is_empty() || scram256.is_empty() {
             return Err(RegError::Internal);
         }
+        // The account may already exist without a verifier — an Anope import
+        // creates accounts (channel ownership, vhosts, etc.) but can't carry the
+        // one-way password hash. When the external authority then asserts a
+        // verifier, set it in place rather than refusing, or migrated users could
+        // never log in. Keyed by name, so all imported data stays attached.
+        if self.exists(name) {
+            self.log
+                .append(Event::AccountPasswordSet {
+                    account: name.to_string(),
+                    scram256: scram256.to_string(),
+                    scram512: scram512.to_string(),
+                })
+                .map_err(|_| RegError::Internal)?;
+            if let Some(acct) = self.accounts.get_mut(&key(name)) {
+                acct.scram256 = Some(scram256.to_string());
+                acct.scram512 = (!scram512.is_empty()).then(|| scram512.to_string());
+            }
+            return Ok(());
+        }
         let account = Account {
             name: name.to_string(),
             email,