Implementation complete:
1. engine/thumbnail.go - New thumbnail rendering package
- GenerateMatchThumbnail() creates 640x360 PNG thumbnails
- Renders grid, bots, cores, walls, energy with player colors
- SelectThumbnailTurn() chooses most interesting turn
- Pure Go stdlib image rendering (no canvas required)
2. cmd/acb-worker - Upload thumbnails to B2 alongside replays
- uploadThumbnail() generates PNG and uploads to B2
- Key: thumbnails/{match_id}.png, content-type: image/png
- Called after match completion, non-blocking on failure
3. cmd/acb-index-builder/deploy.go - Promote thumbnails to R2
- promoteRecentReplays() copies both replays AND thumbnails from B2 to R2
- Thumbnails promoted to warm cache alongside replay promotion
4. cmd/acb-index-builder/generator.go - Populate thumbnail URLs
- buildPlaylistMatch() now includes thumbnail_url field
- URL pattern: https://r2.aicodebattle.com/thumbnails/{match_id}.png
- Enables playlist cards and embed OG tags to show preview images
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
