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/queries/users.sql

@@ -142,3 +142,24 @@ RETURNING *;
 -- to keep the unique index happy and to make the lookup
 -- case-insensitive.
 SELECT * FROM users WHERE lower(email) = lower($1);
+
+-- name: UpdateUserTimezone :exec
+-- Sets the user's IANA timezone and bumps timezone_updated_at. The
+-- handler validates the timezone string via time.LoadLocation before
+-- calling this; the DB does not re-validate.
+UPDATE users
+   SET timezone = $2,
+       timezone_updated_at = now()
+ WHERE id = $1;
+
+-- name: ListActiveUsersWithTimezones :many
+-- Returns (id, timezone) for every user with a play in the last 7
+-- days. The system-playlist scheduler iterates this list at startup
+-- and during its hourly reconciliation to discover newly-active /
+-- no-longer-active users.
+SELECT u.id, u.timezone FROM users u
+ WHERE EXISTS (
+     SELECT 1 FROM play_events pe
+      WHERE pe.user_id = u.id
+        AND pe.started_at > now() - INTERVAL '7 days'
+   );