Rework Authentication into separate services

[jtojnar]

This is a from-scratch rewrite, moving a bit closer to Single Responsibility Principle.

We split handling of credentials-in-config and always-open authentication systems. In the future, we will be able implement more methods this way.

This was motivated by session code being called in constructor would break in CLI with Tracy strict mode.

Additionally:

• Session verification now also checks if the credentials in the config did not change.
• Requests from local IP now give full access to all operations, not just update.

[netlify[bot]]

✅ Deploy Preview for selfoss canceled.

Name	Link
🔨 Latest commit	cf74581ca66466ba205bf760ebe36821ba9b5360
🔍 Latest deploy log	https://app.netlify.com/sites/selfoss/deploys/67433f5ea2c3e30008152c8b