feat(server): add repository prefix path configuration and service struct

- Add REPO_PREFIX_PATH environment variable support in Dockerfile and main.rs
- Introduce GitksService struct with repo_prefix field to manage repository paths
- Implement resolve and resolve_for_init methods for repository path handling
- Add path traversal protection and validation for repository operations
- Update all service implementations to use self.resolve instead of global resolve
- Modify serve function to accept repo_prefix parameter and pass to GitksService
- Remove global resolve functions and integrate them into GitksService struct
- Add proper initialization of repo directory from environment variable

main.rs

@@ -1,3 +1,5 @@
+use std::path::PathBuf;
+
 use gitks::server::serve;
 
 const DEFAULT_HOST: &str = "0.0.0.0";
@@ -7,10 +9,29 @@ const DEFAULT_PORT: &str = "50051";
 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 repo_prefix = std::env::var("REPO_PREFIX_PATH").map_err(|_| {
+        "REPO_PREFIX_PATH environment variable is required (e.g. /data/repos)"
+    })?;
+    let repo_prefix = PathBuf::from(&repo_prefix);
+    if !repo_prefix.is_absolute() {
+        return Err("REPO_PREFIX_PATH must be an absolute path".into());
+    }
+    if !repo_prefix.exists() {
+        std::fs::create_dir_all(&repo_prefix)?;
+    }
+
     let addr: std::net::SocketAddr = format!("{host}:{port}").parse()?;
-    tracing::info!("starting gitks gRPC server on {addr}");
-    serve(addr).await?;
+
+    tracing::info!(
+        "starting gitks gRPC server on {addr}, repo prefix: {}",
+        repo_prefix.display()
+    );
+
+    serve(addr, repo_prefix).await?;
+
     Ok(())
 }