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/fa-IR.json

@@ -266,6 +266,7 @@
         "add": "افزودن فال‌بک",
         "pickInbound": "یک اینباند انتخاب کنید",
         "matchAny": "همه",
+        "destPlaceholder": "خودکار (listen:port فرزند)",
         "rederive": "پر کردن مجدد از فرزند",
         "rederived": "از فرزند پر شد",
         "editAdvanced": "ویرایش فیلدهای مسیریابی",