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/api/rest.rs

@@ -113,8 +113,8 @@ mod tests {
     use super::*;
     use crate::router::MemoryHandler;
     use crate::state::MemoryState;
-    use std::sync::atomic::AtomicUsize;
     use std::sync::RwLock;
+    use std::sync::atomic::AtomicUsize;
     use tempfile::tempdir;
 
     #[tokio::test]
@@ -137,7 +137,9 @@ mod tests {
             block: None,
             reason: None,
         };
-        let res_set = gate_set_handler(State(app_state.clone()), Json(set_req)).await.unwrap();
+        let res_set = gate_set_handler(State(app_state.clone()), Json(set_req))
+            .await
+            .unwrap();
         assert_eq!(res_set.into_response().status(), axum::http::StatusCode::OK);
 
         // Verify the gate (and consume it)
@@ -148,8 +150,13 @@ mod tests {
             params: HashMap::new(),
             consume: true,
         };
-        let res_verify = gate_verify_handler(State(app_state.clone()), Query(verify_req)).await.unwrap();
-        assert_eq!(res_verify.into_response().status(), axum::http::StatusCode::OK);
+        let res_verify = gate_verify_handler(State(app_state.clone()), Query(verify_req))
+            .await
+            .unwrap();
+        assert_eq!(
+            res_verify.into_response().status(),
+            axum::http::StatusCode::OK
+        );
 
         // Verify again should fail since it was consumed
         let verify_req2 = GateVerifyReq {