Add GET /api/progress/all, which returns the signed-in user's progress
and merged watch segments for every video (or one playlist) using two
queries.
- Dashboard: replaces two sequential requests per video with a single
request on load
- Player sidebar: replaces one request per playlist video with a single
request per playlist
Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>