db: purge all references to a dropped account

Dropping (or expiring) an account left its channel access, channel
successorship, and group membership/foundership behind, keyed by name — so
re-registering the same nick silently inherited the old privileges. The
AccountDropped fold and the live drop_account path now share one helper
that erases every reference (founder channels are still handled by
handle_account_gone's successor transfer). Closes a privilege-inheritance
hole; especially relevant to inactivity expiry.

src/engine/db/account.rs

@@ -500,6 +500,10 @@ impl Db {
         }
         self.log.append(Event::AccountDropped { account: account.to_string() }).map_err(|_| RegError::Internal)?;
         self.accounts.remove(&k);
+        // Keep live state in step with what the AccountDropped fold does on replay:
+        // erase every reference so a re-registration can't inherit the account's
+        // channel access, successorship, or group standing.
+        super::event::purge_account_refs(account, &mut self.channels, &mut self.grouped, &mut self.net.groups);
         Ok(true)
     }