Add a 1200x630 social card (dark bg, monospace, teal database-table
motif, the relplay/RDBMS reveal) shown when pages are shared. Source
SVG in src/assets/og-card.svg, rasterised to public/og-card.png with
sharp. Wire site-wide og:image + twitter summary_large_image tags via
Starlight head, with the absolute relplay.org URL.
claude@clouddev1
