perf(server): offload sync disk/db IO to tokio blocking thread pool

Makes apply_sync_write, write_to_local_delta, and store modify truly async, preventing the mcp-memory-server from locking the tokio executor during disk IO

server/src/store.rs

@@ -10,7 +10,7 @@ pub struct Store<T> {
     pub cache: RwLock<T>,
 }
 
-impl<T: DeserializeOwned + Default + Serialize + Clone> Store<T> {
+impl<T: DeserializeOwned + Default + Serialize + Clone + Send + 'static> Store<T> {
     pub fn new(key: &str, db: Arc<Database>) -> Self {
         let initial_data = Self::load_from_db(key, &db);
         Self {
@@ -50,6 +50,11 @@ impl<T: DeserializeOwned + Default + Serialize + Clone> Store<T> {
     pub fn modify<F: FnOnce(&mut T)>(&self, f: F) {
         let mut lock = self.cache.write().unwrap();
         f(&mut lock);
-        Self::save_to_db(&self.key, &self.db, &lock);
+        let key = self.key.clone();
+        let db = self.db.clone();
+        let data = lock.clone();
+        tokio::task::spawn_blocking(move || {
+            Self::save_to_db(&key, &db, &data);
+        });
     }
 }