refactor: apply rust best practices and fix memory optimizations

- Optimized memory allocation in router.rs by offloading JSON serialization to spawn_blocking and using references.
- Prevented full graph duplication on startup in state.rs index rebuild.
- Eliminated massive String allocations in QueryGraphPathHandler BFS loops.
- Avoided temporary Strings in VisualizeGraphHandler via inline writing.
- Fixed O(N) full-graph deduplication in MergeEntitiesHandler to scale efficiently.

server/src/error.rs

@@ -41,8 +41,8 @@ impl IntoResponse for AppError {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use axum::response::IntoResponse;
     use axum::http::StatusCode;
+    use axum::response::IntoResponse;
 
     #[test]
     fn test_app_error_not_found() {
@@ -76,15 +76,14 @@ mod tests {
     fn test_app_error_display() {
         let err = AppError::NotFound("test".into());
         assert_eq!(err.to_string(), "Not Found: test");
-        
+
         let err = AppError::Forbidden("test".into());
         assert_eq!(err.to_string(), "Forbidden: test");
-        
+
         let err = AppError::Internal("test".into());
         assert_eq!(err.to_string(), "Internal Server Error: test");
-        
+
         let err = AppError::BadRequest("test".into());
         assert_eq!(err.to_string(), "Bad Request: test");
     }
 }
-