The following should work for redirects everywhere:
Visit Backlight’s settings.
Click on “Show Advanced Settings” at the bottom of the page
Click on ‘Edit Settings’
Find the setting for “Album .htaccess”
Edit the setting to add in the redirect
Click Save
Visit Backlight > Special Links
Click on “Update Album Files”
Click on the “Update” button
Include the redirect in .htaccess at the top of your side (e.g. under public_html or equivalent), as it looks like you’ve done.
Do the save for backlight/.htaccess
For completeness, you’d want the following to be in place:
The Site URL setting in Backlight’s settings to include the www version of your domain
Your API URL in Lightroom Publisher to include the www version of your domain
Apache to be configured to support both the www and non-www version of your domain
See how you go with those steps and please let us know if anything doesn’t work as it should.
I did get encounter an odd redirect immediately after making the change for a browser tab that was already pointing to Backlight’s admin - my Backlight admin tried loading from /admin instead of /backlight/admin. Manually visiting the URL with /backlight fixed it.
I followed your instructions and wrote exactly the same redirection (that is working perfectly in my root directory) in the backlight advanced settings: