Fixes#7125 and #7069 by implementing a two-step deletion process
that avoids MySQL's restriction on deleting from a table while
selecting from it in a subquery.
The fix separates the SELECT and DELETE operations:
1. First query: Get card IDs for assignments to delete
2. Second query: Delete assignments using the collected IDs
This approach works on all supported database systems (MySQL 5.7+,
MySQL 8.0+, MariaDB 10.x+) and follows MySQL's official best practices
for handling Error 1093: 'You can't specify target table for update in FROM clause'.
The issue occurred because the original deleteByParticipantOnBoard method
used a subquery that referenced the same table being deleted from,
which MySQL prohibits but MariaDB allows (explaining why it worked
in development but failed in production).
Signed-off-by: Arne Bartelt <arne.bartelt@gmail.com>
Signed-off-by: Arne Bartelt <Arne.Bartelt@gmail.com>
