fix: resolve MySQL Error 1093 when deleting users from boards

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>

AUTHORS.md

@@ -6,6 +6,7 @@
 
 - Adrian Missy <adrian.missy@onewavestudios.com>
 - Alexandru Puiu <alexpuiu20@yahoo.com>
+- Arne Bartelt <arne.bartelt@gmail.com>
 - Chandi Langecker <git@chandi.it>
 - Christoph Wurst <christoph@winzerhof-wurst.at>
 - Gary Kim <gary@garykim.dev>