feat(server): add gitks gRPC server with environment configuration

- Add dotenvy dependency for environment variable loading
- Integrate tracing-subscriber for structured logging
- Create main.rs entry point with async server initialization
- Implement environment-based host and port configuration
- Set default address to 0.0.0:50051 for gRPC server
- Add proper error handling with Box<dyn std::error::Error>

main.rs

@@ -0,0 +1,16 @@
+use gitks::server::serve;
+
+const DEFAULT_HOST: &str = "0.0.0.0";
+const DEFAULT_PORT: &str = "50051";
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    dotenvy::dotenv().ok();
+    tracing_subscriber::fmt::init();
+    let host = std::env::var("GITKS_HOST").unwrap_or_else(|_| DEFAULT_HOST.into());
+    let port = std::env::var("GITKS_PORT").unwrap_or_else(|_| DEFAULT_PORT.into());
+    let addr: std::net::SocketAddr = format!("{host}:{port}").parse()?;
+    tracing::info!("starting gitks gRPC server on {addr}");
+    serve(addr).await?;
+    Ok(())
+}