Introduce PresenceTracker interface for online-user tracking

Replace the leaky *redis.Client field on Server with a PresenceTracker
interface that receives connect/rename/disconnect lifecycle events. This
removes Redis-specific set encoding from hotline/server.go and the session
handlers, and drops the redis dependency from the hotline package.

The Redis implementation moves to mobius.RedisPresenceTracker, which owns
the legacy "login::ip"/"login:nickname:ip" set encoding so existing
deployments keep working. The API server reads online users through a new
OnlineLister interface and falls back to the in-memory ClientMgr when no
presence tracker is configured. Startup clearing of stale online state now
happens unconditionally when Redis is configured, not only when the API
server is enabled.

1 files changed, 10 insertions, 2 deletions

diff --git a/cmd/mobius-hotline-server/main.go b/cmd/mobius-hotline-server/main.go
index ba7924d..5191805 100644
--- a/cmd/mobius-hotline-server/main.go
+++ b/cmd/mobius-hotline-server/main.go
@@ -122,6 +122,7 @@ func main() {
 	reloaders = append(reloaders, namedReloader{"message board", messageBoard})
 
 	// Initialize ban list - use Redis if configured, otherwise use file-based storage
+	var onlineLister mobius.OnlineLister
 	if *redisAddr != "" {
 		redisClient := redis.NewClient(&redis.Options{
 			Addr:     *redisAddr,
@@ -135,7 +136,14 @@ func main() {
 			os.Exit(1)
 		}
 
-		srv.Redis = redisClient
+		presence := mobius.NewRedisPresenceTracker(redisClient, slogger)
+		// Discard stale online state from a previous run.
+		if err := presence.Clear(ctx); err != nil {
+			slogger.Warn("Failed to clear online users in Redis", "err", err)
+		}
+		srv.Presence = presence
+		onlineLister = presence
+
 		srv.BanList = mobius.NewRedisBanMgr(redisClient, slogger)
 		slogger.Debug("Using Redis for ban management", "addr", *redisAddr)
 	} else {
@@ -195,7 +203,7 @@ func main() {
 	}
 
 	if *apiAddr != "" {
-		sh := mobius.NewAPIServer(srv, reloadFunc, slogger, *apiKey)
+		sh := mobius.NewAPIServer(srv, onlineLister, reloadFunc, slogger, *apiKey)
 		go sh.Serve(*apiAddr)
 	}