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/models.go

@@ -597,4 +597,5 @@ type Users struct {
 	Timezone          pgtype.Text        `db:"timezone" json:"timezone"`
 	UpdatedAt         pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 	DefaultAudioSpeed float32            `db:"default_audio_speed" json:"default_audio_speed"`
+	KeepScreenAwake   bool               `db:"keep_screen_awake" json:"keep_screen_awake"`
 }