feat: make summary system prompt configurable

[Abhijeet-035]

Summary

• add an admin-configurable summary system prompt
• preserve the existing default prompt when the setting is empty
• use the configured prompt in SummaryProvider while retaining format and complexity instructions
• add unit test coverage for the custom summary system prompt

Closes #405

[lukasdotcom]

Hi @Abhijeet-035,

Thanks for the pr. There was a large pr recently, #431, Could you make sure to remove all the unrelated changes that probably came from that.

[lukasdotcom]

Hi @Abhijeet-035,

You seem to have conflicts now.

[lukasdotcom]

I've fixed it up for you.

[github-actions]

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