feat(profile): keep_screen_awake user preference (default on)

New users column surfaced as an On/Off dropdown in the profile form.
The form and API carry "true"/"false" strings with the theme-style
""-guard, so JSON clients that omit the field can't clobber it;
anything else is rejected. GET/PUT /api/auth/profile expose
keep_screen_awake alongside the other profile fields, and the reader
route threads the preference into its template metadata.

Integration tests pin the schema default, both values, omission
semantics, and rejection — dedicated test user, never testuser.

internal/database/queries/queries.sql

@@ -29,6 +29,7 @@ SELECT
     u.updated_at,
     u.timezone,
     u.default_audio_speed,
+    u.keep_screen_awake,
     (SELECT COUNT(*) FROM devices WHERE user_id = u.id) as device_count
 FROM users u
 WHERE u.id = $1;
@@ -515,6 +516,9 @@ UPDATE users SET timezone = $2, updated_at = NOW() WHERE id = $1;
 -- name: UpdateUserDefaultAudioSpeed :exec
 UPDATE users SET default_audio_speed = $2, updated_at = NOW() WHERE id = $1;
 
+-- name: UpdateUserKeepScreenAwake :exec
+UPDATE users SET keep_screen_awake = $2, updated_at = NOW() WHERE id = $1;
+
 -- Per-book playback speed preference (player plan §8).
 
 -- name: GetAudioPlaybackSpeed :one