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/del.rs

@@ -0,0 +1,27 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// DEL <num>|ALL: remove a memo (or the whole mailbox).
+pub fn handle(me: &str, from: &Sender, account: &str, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    match args.get(1).map(|s| s.to_ascii_uppercase()).as_deref() {
+        Some("ALL") => {
+            let count = db.memo_list(account).len();
+            // Delete from the end so earlier indices stay valid as we go.
+            for i in (0..count).rev() {
+                db.memo_del(account, i);
+            }
+            ctx.notice(me, from.uid, format!("Deleted all \x02{count}\x02 memo(s)."));
+        }
+        Some(numstr) => {
+            let Some(n) = numstr.parse::<usize>().ok().filter(|n| *n >= 1) else {
+                ctx.notice(me, from.uid, "Syntax: DEL <num>|ALL");
+                return;
+            };
+            if db.memo_del(account, n - 1) {
+                ctx.notice(me, from.uid, format!("Memo #\x02{n}\x02 deleted."));
+            } else {
+                ctx.notice(me, from.uid, format!("You have no memo #\x02{n}\x02."));
+            }
+        }
+        None => ctx.notice(me, from.uid, "Syntax: DEL <num>|ALL"),
+    }
+}