Fix export_chat to scan all message DBs

[zhenian]

WCDB writes new messages to the highest-numbered message DB (e.g. message_7), but export_chat.py only read message_0..3, so incremental exports missed new messages. Scan message_*.db via glob like vault_cli.

[mcncarl]

感谢这个修复。我们已经验证：把消息库固定为 message_0..3 确实会在数据库继续分库增长时漏读，技术预检通过。

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[zhenian]

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