Schema + endpoint scaffolding for #392 Half B (per-user timezone
scheduling). Adds two columns to the users table:
- timezone text NOT NULL DEFAULT 'UTC' (IANA name)
- timezone_updated_at timestamptz (nullable; populated on each PUT)
PUT /api/me/timezone validates the IANA value via time.LoadLocation
and writes the row. No scheduler integration yet — the scheduler
struct lands in the next commit and the handler-side Refresh call
in the commit after.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
