feat(outbounds): support custom subscription user agents (#6398)

Some subscription providers require a client-specific User-Agent before returning outbound links. Persist an optional value per subscription and use it for refreshes and previews while preserving the existing default for blank values.

frontend/public/openapi.json

@@ -13569,6 +13569,11 @@
                     "type": "string",
                     "description": "Prefix for generated outbound tags. Defaults to the lowest free \"sub<N>-\" prefix."
                   },
+                  "userAgent": {
+                    "type": "string",
+                    "default": "3x-ui-outbound-sub/1.0",
+                    "description": "Custom User-Agent sent when fetching this subscription. Defaults to \"3x-ui-outbound-sub/1.0\"."
+                  },
                   "updateInterval": {
                     "type": "integer",
                     "default": 600,
@@ -13662,6 +13667,11 @@
                     "type": "string",
                     "description": "Prefix for generated outbound tags. Defaults to the lowest free \"sub<N>-\" prefix."
                   },
+                  "userAgent": {
+                    "type": "string",
+                    "default": "3x-ui-outbound-sub/1.0",
+                    "description": "Custom User-Agent sent when fetching this subscription. Defaults to \"3x-ui-outbound-sub/1.0\"."
+                  },
                   "updateInterval": {
                     "type": "integer",
                     "default": 600,
@@ -13914,6 +13924,10 @@
                     "type": "string",
                     "description": "Subscription URL to preview (required)."
                   },
+                  "userAgent": {
+                    "type": "string",
+                    "description": "Custom User-Agent sent while fetching the preview."
+                  },
                   "allowPrivate": {
                     "type": "boolean",
                     "description": "Allow a private/internal/loopback URL. Default false."