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/router/helpers.go

@@ -76,6 +76,7 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		LastName:          utils.PgTextString(userDB.LastName),
 		CreatedAt:         userDB.CreatedAt.Time,
 		DefaultAudioSpeed: float64(userDB.DefaultAudioSpeed),
+		AudioResumeRewind: int(userDB.AudioResumeRewindSeconds),
 		KeepScreenAwake:   userDB.KeepScreenAwake,
 		ShowPodcastsNav:   showPodcasts,
 	}, nil