Add opt-in external-account mode (delegate identity to the website)

By default fedserv owns accounts itself — nothing changes, no external
service required. Set [auth] external = true and an outside authority (the
website) owns identity instead: NickServ REGISTER / DROP / SET
PASSWORD|EMAIL / RESETPASS / CONFIRM / CERT / GROUP and the IRCv3
registration relay are all refused ("managed on the website"), so IRC
can't mint or change a second identity. Login is unchanged — fedserv still
authenticates locally against the accounts the authority pushes in via the
existing gRPC Accounts API, so lookups stay in-memory fast.

fedserv keeps owning all IRC-domain data (channels, access, vhosts, bans,
memos) keyed by the account name in both modes. One config bool, off by
default, so standalone deployments are unaffected.

src/main.rs

@@ -99,6 +99,7 @@ async fn main() -> Result<()> {
     db.scram_iterations = cfg.server.scram_iterations;
     db.set_outbound(gossip_tx.clone());
     db.set_email_enabled(cfg.email.is_some());
+    db.set_external_accounts(cfg.auth.as_ref().is_some_and(|a| a.external));
     if let Some(email) = &cfg.email {
         db.set_email_brand(&email.brand);
         db.set_email_accent(&email.accent);