optimize tunnel counting from O(n) SCAN to O(1)

[Khaleelhabeeb]

Maintain a global atomic counter (tunnel:global:online_count) that
gets incremented/decremented by TunnelRouter when tunnels come
online/offline.

• Added GLOBAL_ONLINE_COUNT_KEY constant to TunnelRouter
• Increment counter in persistTunnelState on successful registration
• Decrement counter in unregisterTunnel
• Updated cron job to read counter with GET instead of SCAN
• Added fallback to SCAN for backward compatibility on first run
  This eliminates the expensive SCAN operation that iterates through
  all Redis keys, critical for scaling to thousands of active tunnels.

[vercel]

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