MemoServ: add CANCEL, CHECK, and INFO

CANCEL recalls the sender's most recent unread memo to a target (one the
recipient already read can't be recalled); CHECK reports whether the last
memo you sent someone has been read; INFO summarises your mailbox. Adds
memo_cancel/memo_check store methods and shares MAX_MEMOS from the module
root instead of duplicating it in send.rs.

modules/memoserv/src/check.rs

@@ -0,0 +1,18 @@
+use echo_api::{human_time, Sender, ServiceCtx, Store};
+
+// CHECK <nick>: has the last memo you sent to <nick> been read yet?
+pub fn handle(me: &str, from: &Sender, account: &str, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    let Some(&target) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: CHECK <nick>");
+        return;
+    };
+    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;
+    };
+    match db.memo_check(&dest, account) {
+        Some((true, ts)) => ctx.notice(me, from.uid, format!("Your last memo to \x02{target}\x02 (sent {}) has been \x02read\x02.", human_time(ts))),
+        Some((false, ts)) => ctx.notice(me, from.uid, format!("Your last memo to \x02{target}\x02 (sent {}) has \x02not\x02 been read yet.", human_time(ts))),
+        None => ctx.notice(me, from.uid, format!("You have no memo on record to \x02{target}\x02.")),
+    }
+}