Promote BotServ BOTSTATS to a StatServ pseudo-service

Stats now have their own service. StatServ answers /msg StatServ <#channel>
(per-channel activity — lines + top talkers, founder-or-admin) and
/msg StatServ SERVER (the shared cross-service counter registry + gauges,
operators only). The shared counter map moved from the engine onto the
network view so both StatServ (via NetView::stat_counters) and the gRPC
Stats API read one source. BotServ's BOTSTATS command is removed; the
per-channel activity is still recorded in the dispatch loop.

statserv/src/channel.rs

@@ -0,0 +1,21 @@
+use fedserv_api::{NetView, Sender, ServiceCtx, Store};
+
+// <#channel>: the lines seen in a channel this session and its top talkers.
+// Founder-or-admin.
+pub fn handle(me: &str, from: &Sender, chan: &str, ctx: &mut ServiceCtx, net: &dyn NetView, db: &mut dyn Store) {
+    if !super::require_channel_admin(me, from, chan, ctx, db) {
+        return;
+    }
+    let Some((lines, top)) = net.channel_activity(chan) else {
+        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."));
+    if top.is_empty() {
+        return;
+    }
+    ctx.notice(me, from.uid, "Top talkers:");
+    for (i, (nick, count)) in top.iter().enumerate() {
+        ctx.notice(me, from.uid, format!("  {}. \x02{nick}\x02 — {count}", i + 1));
+    }
+}