statserv: persist per-channel activity across restart too

The earlier StatsSet fix persisted only the shared namespaced counters. The
per-channel BOTSTATS view (lines seen + top talkers, from /statserv #chan) lived
on the LiveChannel struct, which is rebuilt from each burst — so it reset to zero
on restart. Move it to a name-keyed Network.chan_activity map (independent of live
membership, seedable before the burst) and carry it in the StatsSet snapshot
(new serde-default `channels` field) alongside the counters. Reworded the
notice from 'seen this session' now that it persists.

modules/statserv/src/channel.rs

@@ -10,7 +10,7 @@ pub fn handle(me: &str, from: &Sender, chan: &str, ctx: &mut ServiceCtx, net: &d
         ctx.notice(me, from.uid, format!("No activity recorded for \x02{chan}\x02 yet."));
         return;
     };
-    ctx.notice(me, from.uid, format!("\x02{chan}\x02 — \x02{lines}\x02 line(s) seen this session."));
+    ctx.notice(me, from.uid, format!("\x02{chan}\x02 — \x02{lines}\x02 line(s) seen."));
     if top.is_empty() {
         return;
     }