feat: serve build history web pages

[HotFazz]

Labels: feat, P7, bonus

Create two HTTP endpoints:

• GET /builds — list all past builds (newest first), showing ID, date, branch, status
• GET /builds/{id} — show full details for one build including logs

Return HTML pages (can be minimal/plain).

Acceptance criteria:

• /builds lists all builds with links to individual build pages
• /builds/{id} shows commit SHA, date, status, and full logs
• Document the build list URL in the README
• Works after server reboot