feat: add CLI configuration flags

Add clap-powered --bind, --static-dir, and --data-dir flags for human-run
server configuration. The merge order is now explicit: command-line arguments
win over UPL_* environment variables, which still fall back to the existing
repository-local defaults.

Document the new flags and allow just run to forward arguments to cargo so the
help text can be checked through the normal task runner.

Test Plan:
- just check
- cargo run -- --help

Refs: none

Cargo.toml

@@ -5,6 +5,7 @@ edition = "2024"
 
 [dependencies]
 axum = "0.8.9"
+clap = { version = "4.5.53", features = ["derive"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.150"
 time = { version = "0.3.47", features = ["formatting", "serde"] }