chore: track seed data in git for portable DB rebuilds

Seed JSON files (parsed legislation) are now committed so that
npm run build:db works on any machine without re-ingestion.
Raw source HTML and .db files remain gitignored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -2,7 +2,6 @@ node_modules/
 dist/
 *.db
 data/source/
-data/seed/
 !data/.gitkeep
 .env
 .env.*