Prevent combat log truncation from breaking kill feed table

[demassimo]

Summary

• add a shared serializer that trims combat log payloads without corrupting their JSON structure
• apply the serializer in both SQLite and MySQL adapters so cached combat logs stay parseable when large
• document the new helper module in FILES.md and the module usage report

Testing

• not run (not requested)

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https://chatgpt.com/codex/tasks/task_e_68e975326978833192aaf2a541f55be5