Split GroupServ into one file per command

REGISTER/DROP/INFO/LIST/ADD/DEL/FLAGS each move to their own file; the
two shared guards (login check, can-manage) stay in lib.rs beside the
dispatcher. No behaviour change.

modules/groupserv/src/add.rs

@@ -0,0 +1,26 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// ADD <!group> <account>: add a plain member (no flags). Needs founder or `f`.
+pub fn handle(me: &str, from: &Sender, name: Option<&str>, target: Option<&str>, ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    let Some(acc) = super::account(me, from, ctx) else { return };
+    let (Some(name), Some(target)) = (name, target) else {
+        ctx.notice(me, from.uid, "Syntax: ADD <!group> <account>");
+        return;
+    };
+    let Some(g) = db.group(name) else {
+        ctx.notice(me, from.uid, format!("\x02{name}\x02 isn't registered."));
+        return;
+    };
+    if !super::can_manage(&g, acc) {
+        ctx.notice(me, from.uid, format!("You need the founder or the \x02f\x02 flag to manage \x02{name}\x02."));
+        return;
+    }
+    let Some(canonical) = db.resolve_account(target).map(str::to_string) else {
+        ctx.notice(me, from.uid, format!("\x02{target}\x02 isn't registered."));
+        return;
+    };
+    match db.group_set_flags(name, &canonical, "") {
+        Ok(()) => ctx.notice(me, from.uid, format!("Added \x02{canonical}\x02 to \x02{name}\x02.")),
+        Err(_) => ctx.notice(me, from.uid, "Sorry, that didn't work. Please try again in a moment."),
+    }
+}