Auth: Activate public mode via `PHOTOPRISM_AUTH_MODE="public"`

[danackerson]

1. What is not working as documented?

I've updated my docker-compose.yml accordingly:

--  PHOTOPRISM_PUBLIC: "true" # No authentication required (disables password protection)
++ PHOTOPRISM_AUTH_MODE: "public" # authentication mode (public, password)

2. How can we reproduce it?

Steps to reproduce the behavior:

1. Go to my photoprism website
2. Redirected to /auth/login

3. What behavior do you expect?

No login screen as I chose "public" AUTH_MODE.

4. What could be the cause of your problem?

Using PHOTOPRISM_AUTH_MODE: "public". Reverting to PHOTOPRISM_PUBLIC: "true" works as expected.

6. Which software versions do you use?

(a) PhotoPrism Architecture & Build Number: AMD64, ARM64, ARMv7,...
PhotoPrism® CE
Build 220728-729ddd920

(b) Database Type & Version: MariaDB, MySQL, SQLite,...
MariaDB

(c) Operating System Types & Versions: Linux, Windows, Android,...
macOS
(d) Browser Types & Versions: Firefox, Chrome, Safari on iPhone,...
Chrome on macOS

[graciousgrey]

Related to #2559

[lastzero]

Started a new preview build build for testing. If it works, we can release it today!