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/Cargo.toml

@@ -47,7 +47,7 @@ server = ["dep:irtt-server", "dep:tokio"]
 tui =["dep:irtt-client", "irtt-client/ancillary", "irtt-client/tokio", "dep:irtt-proto", "dep:irtt-stats", "dep:ratatui", "dep:crossterm"]
 
 [dependencies]
-clap = { version = "4.6.6", features = ["derive"] }
+clap = { version = "4.6.6", features = ["derive", "env"] }
 crossterm = { version = "0.29.0", optional = true }
 ctrlc = "3.5.2"
 irtt-client = { workspace = true, optional = true }