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

src/main.rs

@@ -4,7 +4,7 @@ use upl::app::{AppConfig, build_router};
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn Error>> {
-    let config = AppConfig::from_env()?;
+    let config = AppConfig::from_args()?;
     let listener = tokio::net::TcpListener::bind(config.bind_addr).await?;
 
     println!("upl listening on http://{}", listener.local_addr()?);