feat: implement selective routing for "scope_*" users

- Users with "scope_{name}" prefix are routed to upstreams where {name}
  is present in the "scopes" property (comma-separated).
- Strict separation: Scoped upstreams are excluded from general routing, and vice versa.
- Constraint: SOCKS upstreams and DIRECT(`use_middle_proxy =
false`) mode only.

Example:
  User "scope_hello" matches an upstream with `scopes = "world,hello"`

src/config/load.rs

@@ -208,6 +208,8 @@ impl ProxyConfig {
                 upstream_type: UpstreamType::Direct { interface: None },
                 weight: 1,
                 enabled: true,
+                scopes: String::new(),
+                selected_scope: String::new(),
             });
         }
 

src/config/types.rs

@@ -403,6 +403,10 @@ pub struct UpstreamConfig {
     pub weight: u16,
     #[serde(default = "default_true")]
     pub enabled: bool,
+    #[serde(default)]
+    pub scopes: String,
+    #[serde(skip)]
+    pub selected_scope: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]