fix: expose saved progress in history

[Priveetee]

Summary

• Make GET /history expose saved watch progress from /progress in seconds
• Preserve saved progress when adding history entries with progress=0
• Cover the history/progress contract with route tests

Verification

• CI, Coverage, and Docker passed on dev at 9db10be
• Beta Docker image pulled and recreated with docker-compose.dev-mirror.yml
• Runtime beta curl verified /progress.position=12345ms and /history.progress=12s