Update Vikunja to v2.3 with PostgreSQL

[crxssrazr93]

Summary

• Bump Vikunja from 0.24 to 2.3 (major version upgrade to the v2.x rewrite)
• Switch database from MariaDB to PostgreSQL 18 (now recommended by Vikunja upstream)
• Update JWT secret env var from VIKUNJA_SERVICE_JWTSECRET to VIKUNJA_SERVICE_SECRET (breaking change in v2.x)
• Add registration toggle (VIKUNJA_SERVICE_ENABLEREGISTRATION)
• Remove week start setting (now configurable in the Vikunja UI)
• Remove MariaDB root password variable (not needed with PostgreSQL)

References

• Vikunja Docker docs: https://vikunja.io/docs/full-docker-example
• Vikunja config options: https://vikunja.io/docs/config-options
• Docker Hub: https://hub.docker.com/r/vikunja/vikunja/tags

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First of all, thank you for your contribution! 😄

Please note that this repo is mostly for popular apps with thousands of stars and tens of thousands of downloads. If you'd like to add a less popular app, you can always create your own 3rd party repo and add your app there.

☑️ Self Check before Merge

• I have tested the template using the method described in README.md thoroughly
• I have ensured that I put as much default values as possible (except passwords) to ensure minimum effort required for end users to get started.
• I have ensured that I am not using the "latest" tag as this tag is dynamically changing and might break the one-click app. Use a fixed version.
• I have made sure that instructions.start and instructions.end are clear and self-explanatory.
• Icon is added as a png file to the logos directory.
• I've executed the checks if necessary by running npm ci && npm run validate_apps && npm run formatter (If failling run the prettier: npm run formatter-write)
• I will take responsibility addressing any issues that arises as a result of this PR (maintaining this app).

[githubsaturn]

Please check the selfcheck template added above.

[crxssrazr93]

Please check the selfcheck template added above.

This is done, thank you.

[githubsaturn]

Thanks!