Add index for tiles table to improve query performance

[FryeGao]

Summary

This MR adds a unique index on (zoom_level, tile_column, tile_row) in the tiles table of the MBTiles database.

Motivation

• Querying specific tiles by (z, x, y) was previously unindexed, leading to slower lookups.
• Adding this index significantly improves read performance and ensures data uniqueness.

Changes

• Modified run() method in the MBTiles writer to create an index after the tiles table is created:

  cur.execute(
      "CREATE UNIQUE INDEX tile_index ON tiles (zoom_level, tile_column, tile_row);"
  )