feat: implement 'last IP wins' policy for IP limitation (#3735)

- Add timestamp tracking for each client IP address
- Sort IPs by connection time (newest first) instead of alphabetically
- Automatically disconnect old connections when IP limit exceeded
- Keep only the most recent N IPs based on LimitIP setting
- Force disconnection via Xray API (RemoveUser + AddUser)
- Prevents account sharing while allowing legitimate network switching
- Log format: [LIMIT_IP] Email = user@example.com || Disconnecting OLD IP = 1.2.3.4 || Timestamp = 1738521234

This ensures users can seamlessly switch between networks (mobile/WiFi)
and the system maintains connections from their most recent IPs only.

Fixes account sharing prevention for VPN providers selling per-IP licenses.

Co-authored-by: Aung Ye Zaw <zaw.a.y@phluid.world>

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