feat(users): audio_resume_rewind_seconds column

Per-user resume rewind for the audiobook/podcast player: how many
seconds (0/5/10) before the saved position playback starts, both on
open-resume and again on every genuine unpause. Motivated by the car
startup case — exact resume loses the listener's place, and the
PlaybackRequestResolver path (phone UI, Android Auto, media buttons)
all funnel through one start position.

0 = exact resume (default), so the column is inert until the profile
page sets it. Follows the default_audio_speed / keep_screen_awake
precedent: idempotent ALTER in schema.sql (re-runs on every boot),
explicit GetUser SELECT, dedicated :exec update. Handler-side strict
allow-list {0,5,10} lands with the API slice.

internal/database/queries/queries.sql

@@ -30,6 +30,7 @@ SELECT
     u.timezone,
     u.default_audio_speed,
     u.keep_screen_awake,
+    u.audio_resume_rewind_seconds,
     (SELECT COUNT(*) FROM devices WHERE user_id = u.id) as device_count
 FROM users u
 WHERE u.id = $1;
@@ -526,6 +527,9 @@ 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;
 
+-- name: UpdateUserAudioResumeRewind :exec
+UPDATE users SET audio_resume_rewind_seconds = $2, updated_at = NOW() WHERE id = $1;
+
 -- Per-book playback speed preference (player plan §8).
 
 -- name: GetAudioPlaybackSpeed :one