refactor: shared service layer, default IRC port, smaller functions

Wire up service.Service in HTTP handlers and delegate cleanupUser to
svc.BroadcastQuit for consistent quit/part logic across transports.
Default IRC_LISTEN_ADDR to :6667, remove unused import, fix all lint
issues (dogsled, funcorder, wrapcheck, varnamelen, nolintlint).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handlers/api.go

@@ -3547,52 +3547,16 @@ func (hdlr *Handlers) HandleLogout() http.HandlerFunc {
 }
 
 // cleanupUser parts the user from all channels (notifying
-// members) and deletes the session.
+// members) and deletes the session via the shared service
+// layer.
 func (hdlr *Handlers) cleanupUser(
 	ctx context.Context,
 	sessionID int64,
 	nick string,
 ) {
-	channels, _ := hdlr.params.Database.
-		GetSessionChannels(ctx, sessionID)
-
-	notified := map[int64]bool{}
-
-	var quitDBID int64
-
-	if len(channels) > 0 {
-		quitDBID, _, _ = hdlr.params.Database.InsertMessage(
-			ctx, irc.CmdQuit, nick, "",
-			nil, nil, nil,
-		)
-	}
-
-	for _, chanInfo := range channels {
-		memberIDs, _ := hdlr.params.Database.
-			GetChannelMemberIDs(ctx, chanInfo.ID)
-
-		for _, mid := range memberIDs {
-			if mid != sessionID && !notified[mid] {
-				notified[mid] = true
-
-				_ = hdlr.params.Database.EnqueueToSession(
-					ctx, mid, quitDBID,
-				)
-
-				hdlr.broker.Notify(mid)
-			}
-		}
-
-		_ = hdlr.params.Database.PartChannel(
-			ctx, chanInfo.ID, sessionID,
-		)
-
-		_ = hdlr.params.Database.DeleteChannelIfEmpty(
-			ctx, chanInfo.ID,
-		)
-	}
-
-	_ = hdlr.params.Database.DeleteSession(ctx, sessionID)
+	hdlr.svc.BroadcastQuit(
+		ctx, sessionID, nick, "Connection closed",
+	)
 }
 
 // HandleUsersMe returns the current user's session info.