chore(assets): compress dt-memgraph.png (2.3 MB → 446 KB)

Resize from 2940 px wide to 1600 px and palette-quantize to 256 colors,
keeping text/labels readable while bringing the file in line with the
other dt-*.png screenshots used in the README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>