docs(chart): document chart-version <-> backend-version compatibility matrix

[brandonrc]

Summary

The Helm chart on main deploys OpenSearch (artifact-keeper-iac#67), which only works with backend v1.2.0+. The chart README does not currently spell out which chart versions pair with which backend versions, so users mixing chart main/latest with backend 1.1.x (or vice versa) will hit confusing failures.

Background

• artifact-keeper-iac#67 replaced Meilisearch with OpenSearch in the chart on 2026-04-24.
• artifact-keeper#830 made the same change in the backend on the same day (v1.2.0 unreleased).
• Backend release/1.1.x (current stable v1.1.8) still uses Meilisearch.
• Connects to artifact-keeper-iac#73 (versioning lockstep).

Scope

Add a compatibility matrix to charts/artifact-keeper/README.md (and README.md if appropriate) like:

Chart version	Backend version	Search backend
main (unreleased) / 1.2.x	v1.2.0+	OpenSearch
1.1.x (planned tag)	v1.1.x	Meilisearch

Also:

• Spell out in the chart README install section that pulling main/latest requires backend v1.2.0+.
• If a 1.1.x chart tag exists or is planned, link it for users who need to stay on stable backend.
• Update example install snippet at charts/artifact-keeper/README.md:44 so users do not blindly run a command that targets main without realising the backend image they have on hand may not match.

Out of scope: actually publishing the chart repo (tracked in #51) or aligning chart release tags (tracked in #74).

Acceptance criteria

• Compatibility matrix in chart README.
• Install instructions clearly note backend version requirement.
• Example commands consistent with the matrix.

Related

• chore: replace Meilisearch with OpenSearch in Helm chart (companion to artifact-keeper#830) #67 OpenSearch in chart
• Versioning lockstep across backend / web / chart / openSCAP #73 versioning lockstep
• Helm chart release tags align with backend release tags #74 align chart tags with backend
• Helm repo is not a valid repo #51 helm repo not published
• artifact-keeper#830 backend OpenSearch migration