Split BotServ and MemoServ into one-file-per-command modules

Match the NickServ/ChanServ layout: lib.rs keeps only the service struct
and the on_command dispatcher, and each command moves to its own file
bridged with #[path]. No behaviour change.

BotServ: bot.rs, assign.rs.
MemoServ: send.rs, list.rs, read.rs, del.rs.

memoserv/src/send.rs

@@ -0,0 +1,26 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// A full mailbox rejects new memos, so nobody can be flooded.
+const MAX_MEMOS: usize = 30;
+
+// SEND <nick> <text>: leave a memo on a registered account's mailbox.
+pub fn handle(me: &str, from: &Sender, account: &str, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    if args.len() < 3 {
+        ctx.notice(me, from.uid, "Syntax: SEND <nick> <text>");
+        return;
+    }
+    let target = args[1];
+    let text = args[2..].join(" ");
+    let Some(dest) = db.resolve_account(target).map(str::to_string) else {
+        ctx.notice(me, from.uid, format!("\x02{target}\x02 isn't registered."));
+        return;
+    };
+    if db.memo_list(&dest).len() >= MAX_MEMOS {
+        ctx.notice(me, from.uid, format!("\x02{target}\x02's mailbox is full — they'll need to clear some memos first."));
+        return;
+    }
+    match db.memo_send(&dest, account, &text) {
+        Ok(()) => ctx.notice(me, from.uid, format!("Memo sent to \x02{target}\x02.")),
+        Err(_) => ctx.notice(me, from.uid, "Sorry, that didn't work. Please try again in a moment."),
+    }
+}