feat/refactor: filestore feature flag

[Schwehn42]

Description

Wraps the experimental file store (#4600) inside a feature flag.

Changelog

• moved file store code to router.go, so it only gets executed once
• add new flag/env var SCRUMLR_ENABLE_EXPERIMENTAL_AUTH_FILE_SYSTEM_STORE
• use file store if env var is set to true
• add docs

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4617.development.scrumlr.fra.ics.inovex.io
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Deployed Images

• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-301f67f

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-301f67f

[octomind-dev]

🐙 Octomind

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commit sha: 301f67f