Return 409 conflict when POST /maps with taken name. close #758

[horenso]

Summary

Return 409 conflict when creating a new map with existing name instead of a 500 server error. Also add a simple regression test.

Basics

• [x] The PR is rebased with current master
• [x] I added a line to changelog.md
• [x] Details of what I changed are in the commit messages
• [x] References to issues, e.g. close #X, are in the commit messages and changelog
• [ ] The buildserver is happy

Checklist

• [ ] I fully described what my PR does in the documentation
• [ ] I fixed all affected documentation
• [ ] I fixed the introduction tour
• [ ] I wrote migrations in a way that they are compatible with already present data
• [ ] I fixed all affected decisions
• [ ] I added automated tests or a manual test protocol
• [ ] I added code comments, logging, and assertions as appropriate
• [ ] I translated all strings visible to the user
• [ ] I mentioned every code or binary not directly written or done by me in reuse syntax
• [ ] I created left-over issues for things that are still to be done
• [ ] Code is conforming to our Architecture
• [ ] Code is conforming to our Guidelines
• [ ] Code is consistent to our Design Decisions
• [ ] Exceptions to any guidelines are documented

First Time Checklist

• [ ] I have installed and I am using pre-commit hooks
• [ ] I am using Tailwind CSS Linting

Review

• [ ] I've tested the code
• [ ] I've read through the whole code
• [ ] I've read through the whole documentation
• [ ] I've checked conformity to guidelines
• [ ] I've checked conformity to requirements
• [ ] I've checked that the requirements are tested

[horenso]

jenkins build please

[markus2330]

Thx for the review!

I suggest to update https://github.com/ElektraInitiative/PermaplanT/issues/758 and add a frontend task or create a left over issue.

It is better to create new issues (I created #1307), extending old ones is often confusing, especially if it involves different assignee.

[chr-schr]

jenkins build please