feat(fallbacks): add per-rule dest override

Operators can now type an explicit dest (e.g. "8443", "127.0.0.1:8443",
"/dev/shm/x.sock") on each fallback row to override the auto-resolved
child listen+port. Empty keeps the existing auto behavior.

Adds the column to inbound_fallbacks (GORM AutoMigrate), threads it
through the panel form, API docs, and translations.

web/translation/ru-RU.json

@@ -266,6 +266,7 @@
         "add": "Добавить фолбэк",
         "pickInbound": "Выберите инбаунд",
         "matchAny": "любой",
+        "destPlaceholder": "авто (listen:порт дочернего)",
         "rederive": "Заполнить из дочернего",
         "rederived": "Заполнено из дочернего",
         "editAdvanced": "Изменить поля маршрутизации",