fix(storage): defer concurrent SQLite read transactions

server/src/WebDrop.Storage/SqliteChannelStore.cs

@@ -38,7 +38,8 @@ public sealed class SqliteChannelStore : IResourceStore, IDisposable
     {
         using var conn = Open();
         // 显式事务让一次 Read 内的多条查询看到同一份快照
-        using var tx = conn.BeginTransaction();
+        // 读事务延迟到首次访问时才取快照，避免并发读预检抢占 SQLite 写锁。
+        using var tx = conn.BeginTransaction(deferred: true);
         return action(new SqliteTx(conn));
     }