Add security and context filter for REST api

[Slartibartfass2]

Closes #583

What I have made

This is the baseline for the REST API. Now clients can register, log in, and so on...
I added an integration test that verifies that the whole workflow works.

Checklist

Either tick or cross out the items that do not apply (using ~~example text~~) and give a reason why the item does not apply.

Author

• I have updated the documentation accordingly and commented my code
  • [ ] I have updated AGENTS.md if the changes affect project structure, commands, tooling, or conventions
• I have manually tested my changes
• I have added tests that prove my fix is effective or that my feature works

Reviewer

• I have checked the changes against the requirements

[github-actions]

Code Coverage

Overall Project	98.23%	🍏

There is no coverage information present for the Files changed

[gitguardian]

⚠️ GitGuardian has uncovered 2 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
24483668	Triggered	Generic Password	d7e524a	src/test/kotlin/se/uulm/snowballr/backend/rest/AuthWorkflowIntegrationTest.kt	View secret
36073591	Triggered	Generic Password	d7e524a	src/test/kotlin/se/uulm/snowballr/backend/rest/AuthWorkflowIntegrationTest.kt	View secret

🛠 Guidelines to remediate hardcoded secrets

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3. Revoke and rotate these secrets.
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• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}