Timestamp Sorting for Faster Data Access

## Current Problem
When we fetch mempool data (transactions), we don't sort transactions by timestamp. This makes it slow to find transactions within specific time ranges.

## Why This Matters
- Searching for transactions in a time range is much slower than it could be
- Each search has to scan through unsorted data
- This affects performance of time-based queries

## Simple Solution
Sort the data by timestamp when saving it. This will make time-based searches much faster. Also we could split it with smaller equal row_group_size like 5000

## Expected Benefits
- Much faster time-range searches

Start by locating the code that saves mempool transactions to Parquet and CSV, then trace how timestamps and row_group_size are handled. Determine how sorted data is queried for time ranges and validate that saving in timestamp order with the proposed row-group size improves those searches without breaking existing output.

Written by the indexing model from the issue text.