SRCH-5887: Added Elasticsearch ILM polices

[igoristic]

This PR adds support for managing Elasticsearch index retention using https://www.elastic.co/guide/en/elasticsearch/reference/current/index-lifecycle-management.html. It ensures that an ILM policy is created or updated, applied to a matching index template, and attached to both new and existing indices.

More details here: https://docs.google.com/document/d/1nhWg50eILV1_tglNJDl-gUk-SBy6D7Gr3raimCygnC4/edit?tab=t.0

Key enhancements:

• Defines a hot+delete ILM policy with configurable retention age.
• Applies the policy on index creation.
• Updates existing indices to attach the ILM policy if not already present.
• Waits for cluster health (yellow or better) after index creation.

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ILM Policy Details

• Hot Phase: Immediate (min_age: 0ms), sets index priority.
• Delete Phase: Triggers at SEARCHELASTIC_INDEX_RETENTION_MIN_AGE, deletes the index.

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How to Test

You can test ILM in a staging or dev environment using a short retention window to verify it’s working as expected.

1. Set a small retention age in your environment (e.g. 1 hour):

   export SEARCHELASTIC_INDEX_RETENTION_MIN_AGE="1h"

2. Create the index by running the Rails method:

   ES.create_index

3. Confirm ILM policy is attached:

   curl -X GET localhost:9200/spider-searchgov/_ilm/explain?pretty

4. After 1 hour, confirm that the index is automatically deleted.

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Note:

• For existing static indices, the script will attach the ILM policy if missing.
• Consider using aliases and reindexing for full ILM migration (see documentation added in this PR).
• Always validate in staging before rolling out to production.

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