fix: catch DoesNotExistException in DeckShareProvider

[grnd-alt]

• Resolves: Deleting a board that has file attachments breaks the filesystem for every board member (unhandled DoesNotExistException in SetupManager) #8433
• Target version: main

Summary

Deleting a Board caused DeckShareProvider to rethrow a NotFoundException from the db query that was not caught.
That caused the file system to not be available until the cronjob actually deleted the board from the database.

We now catch that exception and remove the shares from the list.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

🤖 AI (if applicable)

• [] The content of this PR was partly or fully generated using AI (N/A)

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 98827 was 93102 (+6.14%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.