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/router/helpers.go

@@ -67,6 +67,7 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		LastName:          utils.PgTextString(userDB.LastName),
 		CreatedAt:         userDB.CreatedAt.Time,
 		DefaultAudioSpeed: float64(userDB.DefaultAudioSpeed),
+		KeepScreenAwake:   userDB.KeepScreenAwake,
 	}, nil
 }