feat(app): add environment variable support for server configuration

Enable configuration via IRTT_SERVER_* environment variables to simplify deployments in containerized or orchestrated environments. This adds the clap "env" feature and includes thread-safe tests to verify variable precedence.

crates/irtt-app/src/cmd/server/mod.rs

@@ -1,4 +1,6 @@
 pub mod args;
+#[cfg(test)]
+pub(crate) mod env_lock;
 pub mod run;
 
 pub use args::ServerArgs;