feat(iplimit): gate IP limit on fail2ban and reset stale limits

Per-client IP limit only enforces where fail2ban is installed, so the panel now reports enforceability and disables the field otherwise:

- Add GET /panel/api/server/fail2banStatus (enabled/installed/usable/windows), cached 30s.
- ClientFormModal and ClientBulkAddModal disable the IP Limit input when not usable and show a hover tooltip; Windows gets a platform-specific message instead of the bash-menu hint.
- One-time migration ResetIpLimitNoFail2ban zeroes existing client limitIp (inbound settings JSON + clients table) on hosts without fail2ban, where the limit never applied.
- Drop the recurring '[LimitIP] Fail2Ban is not installed' warning.
- Add limitIpFail2banMissing/limitIpFail2banWindows/limitIpDisabled across all 13 locales.

internal/web/translation/id-ID.json

@@ -747,6 +747,9 @@
       "addClients": "Tambah klien",
       "limitIp": "Batas IP",
       "limitIpDesc": "Jumlah maksimum IP bersamaan. 0 = tidak terbatas.",
+      "limitIpFail2banMissing": "Fail2ban tidak terpasang, sehingga batas IP tidak dapat diterapkan. Pasang Fail2ban dari menu bash x-ui untuk mengaktifkan opsi ini.",
+      "limitIpFail2banWindows": "Fail2ban tidak tersedia di Windows, sehingga batas IP tidak dapat diterapkan.",
+      "limitIpDisabled": "Fitur batas IP dinonaktifkan di server ini.",
       "password": "Kata sandi",
       "subId": "ID Langganan",
       "online": "Online",