Add configurable global concurrent request limiting

- Add max_concurrent_requests config option (default: 1000)
- Implement global AtomicUsize counter for concurrent request tracking
- Return status 41 'Server unavailable' when limit exceeded
- Proper counter management with decrements on all exit paths
- Add comprehensive config validation (1-1,000,000 range)
- Update documentation with rate limiting details
- Add unit tests for config parsing
- Thread-safe implementation using Ordering::Relaxed

This provides effective DDoS protection by limiting concurrent
connections to prevent server overload while maintaining
configurability for different deployment scenarios.

src/main.rs

@@ -69,6 +69,7 @@ async fn main() {
         host: None,
         port: None,
         log_level: None,
+        max_concurrent_requests: None,
     });
 
     // Initialize logging
@@ -82,6 +83,13 @@ async fn main() {
     let host = args.host.or(config.host).unwrap_or_else(|| "0.0.0.0".to_string());
     let port = args.port.or(config.port).unwrap_or(1965);
 
+    // Validate max concurrent requests
+    let max_concurrent_requests = config.max_concurrent_requests.unwrap_or(1000);
+    if max_concurrent_requests == 0 || max_concurrent_requests > 1_000_000 {
+        eprintln!("Error: max_concurrent_requests must be between 1 and 1,000,000");
+        std::process::exit(1);
+    }
+
     // Validate directory
     let dir_path = Path::new(&root);
     if !dir_path.exists() || !dir_path.is_dir() {
@@ -110,8 +118,9 @@ async fn main() {
         let acceptor = acceptor.clone();
         let dir = root.clone();
         let expected_host = "localhost".to_string(); // Override for testing
+        let max_concurrent = max_concurrent_requests;
         if let Ok(stream) = acceptor.accept(stream).await {
-            if let Err(e) = server::handle_connection(stream, &dir, &expected_host).await {
+            if let Err(e) = server::handle_connection(stream, &dir, &expected_host, max_concurrent).await {
                 tracing::error!("Error handling connection: {}", e);
             }
         }