feat(29-04): create backfill script for dominant colors

One-time migration script processes items, globalItems, and
threadCandidates to extract dominant colors via Sharp. Idempotent,
batched (10 concurrent), with progress logging.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -14,7 +14,8 @@
 		"test:e2e": "bunx playwright test",
 		"test:e2e:ui": "bunx playwright test --ui",
 		"lint": "bunx @biomejs/biome check .",
-		"db:seed:dev": "bun run src/db/dev-seed.ts"
+		"db:seed:dev": "bun run src/db/dev-seed.ts",
+		"backfill:colors": "bun run scripts/backfill-dominant-colors.ts"
 	},
 	"devDependencies": {
 		"@biomejs/biome": "^2.4.7",