fix: handle foreign key constraints when deleting QSOs

The qso_changes table has a foreign key reference to qsos.id, which
was preventing QSO deletion. Now deletes related qso_changes records
first before deleting QSOs.

Also added better error logging to the DELETE endpoint.

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/index.js

@@ -867,6 +867,7 @@ const app = new Elysia()
         message: `Deleted ${deleted} QSO(s)`,
       };
     } catch (error) {
+      logger.error('Failed to delete QSOs', { error: error.message, stack: error.stack });
       set.status = 500;
       return {
         success: false,