Add OperServ with AKILL network bans

The first operator module. AKILL manages network bans as a typed,
event-sourced list with lazy expiry, mirroring the patterns the other
modules use:

- AKILL ADD [+expiry] <user@host> <reason> stores the ban and drives an
  ircd G-line (applied to matching users already online and to future
  connections); AKILL DEL <mask|number> lifts it; AKILL LIST [pattern]
  shows the live list, numbered. Admin-only.
- Bans are Global events, so every node holds the list and re-asserts each
  one at burst with its remaining duration. Expired bans are hidden lazily
  and dropped at compaction.
- Masks are normalised to user@host (a nick! prefix is stripped, both
  sides lowercased) and an all-wildcard mask is refused.

Email (send_email) is already reachable from the api via ServiceCtx, and
adds AddLine/DelLine to the action vocabulary for any future X-line kinds.

src/config.rs

@@ -107,7 +107,7 @@ impl Default for Modules {
 }
 
 fn default_services() -> Vec<String> {
-    vec!["nickserv".to_string(), "chanserv".to_string(), "botserv".to_string(), "memoserv".to_string(), "statserv".to_string(), "hostserv".to_string()]
+    vec!["nickserv".to_string(), "chanserv".to_string(), "botserv".to_string(), "memoserv".to_string(), "statserv".to_string(), "hostserv".to_string(), "operserv".to_string()]
 }
 
 #[derive(Debug, Deserialize, Clone)]