feat(users): timezone column + PUT /api/me/timezone

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>

internal/db/migrations/0026_users_timezone.up.sql

@@ -0,0 +1,8 @@
+-- 0026_users_timezone.up.sql — per-user timezone for the system-playlist
+-- scheduler (#392 Half B). Default 'UTC' so existing rows + brand-new
+-- users have a safe value before their first client check-in via
+-- PUT /api/me/timezone. The scheduler reads this column to decide
+-- which IANA zone to anchor each user's daily 03:00 build to.
+ALTER TABLE users
+    ADD COLUMN timezone text NOT NULL DEFAULT 'UTC',
+    ADD COLUMN timezone_updated_at timestamptz;