Persist 'nullified' death bans and add DB support

[Nergly]

Motivation

• Retain death records that do not produce an active death ban by marking them as nullified so history is preserved and consumers can treat them differently.
• Provide database and mapper support for the new flag so persistence and replication work across servers and schema versions.

Description

• Add a nullified boolean to Ban with constructors, isNullified/setNullified, and include it in serialize and Deserialize.
• Always store a death record for banTime == 0 in PlayerData, mark it nullified, and still add/remove it from server data and update the player repository.
• When removing an ongoing ban in ServerData, set nullified to true on the ban before removing.
• Extend MySQLBanMapper to detect whether the nullified column exists, include it in the generated INSERT/UPDATE SQL and parameter binding, and safely read nullified from result sets for backward compatibility; update dbsetup.sql to add the nullified column with default 0.

Testing

• Ran the project's automated test suite and mapper unit tests, which completed successfully.
• Exercised the MySQL mapper against schemas with and without the nullified column to verify detection and backward-compatible reads succeeded.

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Codex Task