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.

api/src/lib.rs

@@ -629,6 +629,9 @@ pub trait Store {
     fn unsuspend_account(&mut self, account: &str) -> Result<bool, RegError>;
     fn is_suspended(&self, account: &str) -> bool;
     fn suspension(&self, account: &str) -> Option<SuspensionView>;
+    // Inactivity-expiry pins (oper-only, gated on Priv::Admin at the command layer).
+    fn set_account_noexpire(&mut self, account: &str, on: bool) -> Result<bool, RegError>;
+    fn set_channel_noexpire(&mut self, channel: &str, on: bool) -> Result<bool, ChanError>;
     fn register_channel(&mut self, name: &str, founder: &str) -> Result<(), ChanError>;
     fn drop_channel(&mut self, name: &str) -> Result<(), ChanError>;
     fn set_mlock(&mut self, name: &str, on: &str, off: &str) -> Result<(), ChanError>;