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.

src/config.rs

@@ -29,6 +29,30 @@ pub struct Config {
     // Staff audit feed. Absent = no audit log is emitted.
     #[serde(default)]
     pub log: Option<Log>,
+    // Inactivity-expiry. Absent = accounts and channels never expire.
+    #[serde(default)]
+    pub expire: Option<Expire>,
+}
+
+// Inactivity-expiry thresholds, in days. A zero (or omitted) field leaves that
+// kind never expiring, so an operator can expire only accounts, only channels,
+// or both.
+#[derive(Debug, Deserialize, Clone)]
+pub struct Expire {
+    #[serde(default)]
+    pub accounts_days: u64,
+    #[serde(default)]
+    pub channels_days: u64,
+}
+
+impl Expire {
+    // The thresholds in seconds, or None where that kind is disabled (zero days).
+    pub fn account_ttl(&self) -> Option<u64> {
+        (self.accounts_days > 0).then(|| self.accounts_days * 86_400)
+    }
+    pub fn channel_ttl(&self) -> Option<u64> {
+        (self.channels_days > 0).then(|| self.channels_days * 86_400)
+    }
 }
 
 // The staff audit feed: notable service actions are announced to this channel