Final documentation and submission cleanup

[FriggemannMichael]

Objective:
Prepare the repository for final submission.

Acceptance criteria:

• README accurately describes setup, checks, and known project behavior.
• requirements.txt is complete and reproducible.
• Database and local files are not tracked.
• No commented-out code, print debugging, or unrelated files remain.
• Ruff, Django checks, and tests pass from the local virtual environment.
• docs/checkliste.md and docs/endpoints.md are reviewed against the implementation.

[FriggemannMichael]

TDD workflow for this project:

• Define the expected behavior before implementing the endpoint or model behavior.
• Write or outline the failing test first.
• Implement only enough code to make that test pass.
• Refactor after the test passes.
• Keep Django check, Ruff, and the relevant tests green from the local virtual environment.