Add de/en languages to the database on startup if there are no languages

[lkeegan]

• ensures the languages database always contains the rows
  • 1 de
  • 2 en
• don't allow admins to delete either of these languages
  • no use case where this would be required
  • and could make a mess in production if it happened by accident
• remove delete button from admin interface for these languages
• resolves #126

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 23.91%. Comparing base (2daf248) to head (115f5a1). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...backend/src/mondey_backend/databases/milestones.py	37.50%	5 Missing :warning:
frontend/src/lib/components/Admin/Languages.svelte	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #127 +/- ## ========================================== + Coverage 17.22% 23.91% +6.69% ========================================== Files 98 95 -3 Lines 4974 3796 -1178 Branches 125 122 -3 ========================================== + Hits 857 908 +51 + Misses 4047 2819 -1228 + Partials 70 69 -1 ```

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[MaHaWo]

discussion IRL:

• first is an issue that arises when starting from a non-empty database
• second issue will become new pr