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, 20 insertions, 0 deletions

diff --git a/hotline/presence.go b/hotline/presence.go
new file mode 100644
index 0000000..7e41357
--- /dev/null
+++ b/hotline/presence.go
@@ -0,0 +1,20 @@
+package hotline
+
+// PresenceTracker receives best-effort notifications about user session lifecycle
+// events so an external system (e.g. Redis) can maintain a list of online users.
+//
+// It is optional: when a Server has no PresenceTracker, online presence is derived
+// from the in-memory ClientManager instead. Implementations must be safe for
+// concurrent use by multiple goroutines.
+type PresenceTracker interface {
+	// UserConnected is called after a successful login, before the nickname is known.
+	UserConnected(login, ip string)
+
+	// UserRenamed is called when a user's nickname is set or changed. oldNickname is
+	// empty the first time a nickname is set (e.g. the 1.2.3 login flow or TranAgreed).
+	UserRenamed(login, oldNickname, newNickname, ip string)
+
+	// UserDisconnected is called when a user's session ends. nickname is empty if the
+	// user disconnected before setting one.
+	UserDisconnected(login, nickname, ip string)
+}