server: scrub a departed user's pending invites via a User.invited reverse index instead of scanning every channel on the network per quit — the old O(channels)-per-quit path was O(channels*quits) on a netsplit; the index is maintained at the 4 invite add/remove sites (INVITE cmd, S2S INVITE, join-consume, UNINVITE)

src/users.rs

@@ -248,6 +248,7 @@ pub struct User {
     pub caps: Caps,            // enabled IRCv3 capabilities
     pub sasl_mech: Option<String>, // SASL mechanism chosen, mid-handshake
     pub channels: HashSet<String>, // lowercased channel keys
+    pub invited: HashSet<String>,  // channels this user has a pending +i invite to (reverse index)
     pub watch: Vec<String>,    // WATCH list — lowercased nicks
     pub monitor: Vec<String>,  // MONITOR list — lowercased nicks
     pub silence: Vec<String>,  // SILENCE masks — nick!user@host globs