Apply [timeouts] tg_connect to upstream DC TCP connect attempts

Wire config.timeouts.tg_connect into UpstreamManager; per-attempt timeout uses
the same .max(1) pattern as connect_budget_ms.

Reject timeouts.tg_connect = 0 at config load (consistent with
general.upstream_connect_budget_ms and related checks). Default when the key
is omitted remains default_connect_timeout() via serde.

Fixes telemt/telemt#439

src/config/load.rs

@@ -346,6 +346,12 @@ impl ProxyConfig {
             ));
         }
 
+        if config.timeouts.tg_connect == 0 {
+            return Err(ProxyError::Config(
+                "timeouts.tg_connect must be > 0".to_string(),
+            ));
+        }
+
         if config.general.upstream_unhealthy_fail_threshold == 0 {
             return Err(ProxyError::Config(
                 "general.upstream_unhealthy_fail_threshold must be > 0".to_string(),
@@ -1907,6 +1913,26 @@ mod tests {
         let _ = std::fs::remove_file(path);
     }
 
+    #[test]
+    fn tg_connect_zero_is_rejected() {
+        let toml = r#"
+            [timeouts]
+            tg_connect = 0
+
+            [censorship]
+            tls_domain = "example.com"
+
+            [access.users]
+            user = "00000000000000000000000000000000"
+        "#;
+        let dir = std::env::temp_dir();
+        let path = dir.join("telemt_tg_connect_zero_test.toml");
+        std::fs::write(&path, toml).unwrap();
+        let err = ProxyConfig::load(&path).unwrap_err().to_string();
+        assert!(err.contains("timeouts.tg_connect must be > 0"));
+        let _ = std::fs::remove_file(path);
+    }
+
     #[test]
     fn rpc_proxy_req_every_out_of_range_is_rejected() {
         let toml = r#"