Adding cached markdown fields to Review LLM Summaries

What does this MR do and why?

This MR adds fields to support cached markdown content for Review LLM Summaries. This is a database precursor for !130221 (merged) that will add the code for it.

How to set up and validate locally

1. Clone the repository and checkout the master branch.
2. Run bundle install.
3. Run rake db:migrate.
4. Confirm the migration runs correctly

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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This description was generated for revision 130357e5 using AI