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/en-US.json

@@ -747,6 +747,9 @@
       "addClients": "Add Clients",
       "limitIp": "IP Limit",
       "limitIpDesc": "Maximum simultaneous IPs. 0 = unlimited.",
+      "limitIpFail2banMissing": "Fail2ban is not installed, so the IP limit cannot be enforced. Install Fail2ban from the x-ui bash menu to enable this option.",
+      "limitIpFail2banWindows": "Fail2ban is not available on Windows, so the IP limit cannot be enforced.",
+      "limitIpDisabled": "The IP limit feature is disabled on this server.",
       "password": "Password",
       "subId": "Subscription ID",
       "online": "Online",