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.

frontend/src/schemas/forms/inbound-form.ts

@@ -78,6 +78,7 @@ export const FallbackRowSchema = z.object({
   name: z.string().default(''),
   alpn: z.string().default(''),
   path: z.string().default(''),
+  dest: z.string().default(''),
   xver: z.number().int().min(0).max(2).default(0),
 });
 export type FallbackRow = z.infer<typeof FallbackRowSchema>;