Add lazy inactivity-expiry for accounts and channels

Accounts and channels now carry a last-activity stamp and are dropped
once inactive past a configured threshold. A login stamps the account, a
join stamps the channel — both coalesced to at most one log entry per day
so routine traffic doesn't bloat the log, and stamps replicate so
activity on any node keeps a record alive network-wide.

Expiry is lazy in the proven style: a periodic pass computes it from the
stored stamps with no per-record timer, reusing the same relation the
vhost and suspension expiries use. The sweep spares operator accounts,
accounts with a live session, occupied channels, and anything an operator
has pinned with the new NOEXPIRE command (NickServ NOEXPIRE <account>,
ChanServ NOEXPIRE <#channel>, both Priv::Admin). Each expiry is announced
to the audit channel and clears the channel's +r.

Configured under [expire] (accounts_days / channels_days); a zero or
omitted field leaves that kind never expiring, and omitting the section
disables expiry entirely.

nickserv/src/noexpire.rs

@@ -0,0 +1,33 @@
+use fedserv_api::{Priv, Sender, ServiceCtx, Store};
+
+// NOEXPIRE <account> {ON|OFF}: pin an account so inactivity-expiry never drops
+// it (or lift the pin). Oper-only (Priv::Admin) — protecting a record from
+// expiry is a staff decision, not self-service.
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    if !from.privs.has(Priv::Admin) {
+        ctx.notice(me, from.uid, "Access denied — that command is for services operators.");
+        return;
+    }
+    let (Some(&target), Some(on)) = (args.get(1), args.get(2).and_then(|s| parse_toggle(s))) else {
+        ctx.notice(me, from.uid, "Syntax: NOEXPIRE <account> {ON|OFF}");
+        return;
+    };
+    let Some(account) = db.resolve_account(target).map(str::to_string) else {
+        ctx.notice(me, from.uid, format!("\x02{target}\x02 isn't registered."));
+        return;
+    };
+    match db.set_account_noexpire(&account, on) {
+        Ok(true) if on => ctx.notice(me, from.uid, format!("\x02{account}\x02 will no longer expire.")),
+        Ok(true) => ctx.notice(me, from.uid, format!("\x02{account}\x02 can expire from inactivity again.")),
+        Ok(false) => ctx.notice(me, from.uid, format!("\x02{account}\x02 was already set that way.")),
+        Err(_) => ctx.notice(me, from.uid, "Sorry, that didn't work. Please try again in a moment."),
+    }
+}
+
+fn parse_toggle(s: &str) -> Option<bool> {
+    match s.to_ascii_uppercase().as_str() {
+        "ON" | "TRUE" | "YES" => Some(true),
+        "OFF" | "FALSE" | "NO" => Some(false),
+        _ => None,
+    }
+}