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/maestro/mod.rs

@@ -223,6 +223,7 @@ pub async fn run() -> std::result::Result<(), Box<dyn std::error::Error>> {
         config.general.upstream_connect_retry_attempts,
         config.general.upstream_connect_retry_backoff_ms,
         config.general.upstream_connect_budget_ms,
+        config.timeouts.tg_connect,
         config.general.upstream_unhealthy_fail_threshold,
         config.general.upstream_connect_failfast_hard_errors,
         stats.clone(),