Post-fix: Unify public URL config

[byewokko]

post-fix to #328

Summary

• Simplify public URL configuration to just one option: public_url in the general section.
• Public server URL can be also supplied via PUBLIC_URL environment variable.
• All necessary public URLs (login page, public API, OpenID Connect API, WebAuthn issuer name ...) are derived from public_url in canonical deployments.

Breaking change guide

• Remove the obsolete config option public_api_base_url.
• Add option public_url in the [general] config section. In most cases, its value should be the same as public_api_base_url but without the path component (e.g. public_url=https://example.com instead of public_url=https://example.com/auth/api). Or you can set the public URL value using the PUBLIC_URL environment variable, if you prefer.
• If your auth_webui_base_url path is /auth, you can remove this option because /auth is the default value.
• If your deployment does not use canonical paths (for Seacat Auth APIs and/or login UI), you can specify your custom paths using the options public_seacat_auth_base_url and public_openidconnect_base_url.

The minimal recommended config can look like

[general]
public_url=https://auth.example.com/

which is equivalent to setting PUBLIC_URL=https://auth.example.com.

For non-canonical deployments the config may be for example

[general]
public_url=https://auth.example.com/
public_seacat_auth_base_url=/seacat/
public_openidconnect_base_url=/
auth_webui_base_url=https://login.example.com/

---

Configuration details

[general] public_url

• Provides base URL from which all the other public relative URLs are derived.
• Can be also configured by PUBLIC_URL environment variable.
• Defaults to http://localhost (but produces a warning on app initialization if the default is used).

[general] public_seacat_auth_base_url

• Seacat Auth API URL prefix.
• Used for example for generating callback URIs for external login.
• Defaults to canonical path api/seacat-auth/ (which is expanded to ${PUBLIC_URL}/api/seacat-auth/).
• Endpoint URL example https://example.com/api/seacat-auth/public/login.

[general] public_openidconnect_base_url

• OpenID Connect API URL prefix.
• Used for example for generating OpenID Connect discovery URLs (authorization endpoint, token endpoint etc.).
• Defaults to canonical path api/ (which is expanded to ${PUBLIC_URL}/api/)
• Endpoint URL example https://example.com/api/openidconnect/authorize.

[general] auth_webui_base_url

• Seacat Auth WebUI URL.
• Used for example for redirecting the user to the login page.
• Defaults to canonical path auth/ (which is expanded to ${PUBLIC_URL}/auth/).
• Page URL example https://example.com/auth/#/login.