feat: implement connection caching for ghost user lifecycle management

Implement comprehensive connection caching system to prevent XMPP connection leaks and support HA environments:

- Add node-specific XMPP resources using format "{baseResource}-node-{diagnosticId[:8]}" for HA compatibility
- Implement thread-safe connection cache with mutex protection in UserManager
- Add cache-first lookup in GetUser/GetOrCreateUser methods to prevent duplicate connections
- Update lifecycle manager to efficiently check cached users instead of expensive KV store queries
- Add graceful shutdown cleanup to properly disconnect all cached connections
- Implement cache management methods: getCachedUser, cacheUser, removeCachedUser, getCachedUsers
- Update activity tracking to work with cached connections
- Add proper cache cleanup when users are disconnected or deleted

This prevents connection leaks identified in previous implementation while maintaining efficient ghost user lifecycle management with 30-minute inactivity timeout.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/bridge/xmpp/message_handler.go

@@ -123,6 +123,12 @@ func (h *xmppMessageHandler) sendMessageViaGhostUser(xmppUserManager *UserManage
 		return h.sendMessageViaBridgeUser(msg, roomJID)
 	}
 
+	// Update user activity in KV store after successful message send
+	if err := xmppUserManager.UpdateUserActivity(msg.SourceUserID); err != nil {
+		h.logger.LogError("Failed to update user activity after message send, user may never disconnect", "user_id", msg.SourceUserID, "error", err)
+		// Don't fail the message send for activity update failures
+	}
+
 	h.logger.LogDebug("Message sent via ghost user",
 		"source_user_id", msg.SourceUserID,
 		"ghost_jid", xmppUser.GetJID(),