feat(api): expose and validate the audio resume rewind setting

GET/PUT /api/auth/profile now carries audio_resume_rewind_seconds
(always-present on the response: 0 is a real value, exact resume; the
request shape is the keep_screen_awake string precedent so the form
select can send an explicit 0=Off while JSON clients that omit it
leave the column untouched — a plain int with an omitempty contract
could not express 'set to off'). Strict allow-list {0,5,10}: the UI
offers exactly three choices and the API refuses everything else.

The audiobook chapters endpoint embeds resume_rewind_seconds in its
existing payload — it already loads the user row for playback speed,
so both players (web + Android, phone UI and the Android Auto service)
get the value with the fetch they perform anyway; zero extra round
trips.

Profile page gains the Resume Rewind select (Off / 5 s before /
10 s before) next to Default Playback Speed.

internal/handlers/media.go

@@ -3101,13 +3101,18 @@ func (mh *MediaHandler) GetAudioChapters(c *echo.Context) error {
 	// 1.0 IN the payload so the player applies it with zero extra round
 	// trips; default_playback_speed powers the dropdown's "(default)"
 	// marker. Speed is a preference, so this rides the authenticated
-	// user — anonymous readers just get 1.0.
+	// user — anonymous readers just get 1.0. The resume rewind rides
+	// the same user row (0 = exact resume).
 	playbackSpeed, defaultSpeed := 1.0, 1.0
+	resumeRewind := 0
 	if userID, ok := c.Get("user_id").(string); ok {
 		if userUUID, err := uuid.Parse(userID); err == nil {
 			userRow, err := mh.db.GetUser(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
-			if err == nil && userRow.DefaultAudioSpeed > 0 {
-				defaultSpeed = float64(userRow.DefaultAudioSpeed)
+			if err == nil {
+				if userRow.DefaultAudioSpeed > 0 {
+					defaultSpeed = float64(userRow.DefaultAudioSpeed)
+				}
+				resumeRewind = int(userRow.AudioResumeRewindSeconds)
 			}
 			bookSpeed, err := mh.db.GetAudioPlaybackSpeed(c.Request().Context(), database.GetAudioPlaybackSpeedParams{
 				UserID:      pgtype.UUID{Bytes: userUUID, Valid: true},
@@ -3130,6 +3135,7 @@ func (mh *MediaHandler) GetAudioChapters(c *echo.Context) error {
 		"tracks":                 tracks,
 		"playback_speed":         playbackSpeed,
 		"default_playback_speed": defaultSpeed,
+		"resume_rewind_seconds":  resumeRewind,
 	})
 }