/schedule page + slot builder

[quinn-dougherty]

Description

Publishable schedule derived from `schedule_slots` + accepted `submissions`. PLAYBOOK.md §1, §6.

Requirements

• `/schedule` route — static placeholder until published 2026-10-01.
• `/admin/schedule` — drag-or-form builder to assign `submission_id` to `schedule_slots` rows (start_at, end_at, room).
• Render canonical timezone, with client-side local-time toggle.
• Pre-event reminder email (T-7) links back here.

Dependencies

• Submissions decided (Build /submit form (papers / repos / breakout sessions) #1).
• Admin auth (admin issue).