conf(compose & chart): upgrade backend to 2.40.2 & bump chart

[thomasboni]

No description provided.

[cursor]

PR Summary

Low Risk
Version pin bumps only; no application logic or deployment structure changes in this diff.

Overview
Bumps the Plumber backend container image from v2.40.1 to v2.40.2 for both the API (backend) and job processor (worker) in the Helm chart, and aligns the shared BACKEND_IMAGE_TAG in versions.env (used by Docker Compose via ${BACKEND_IMAGE_TAG}).

The Helm chart release is incremented to 1.3.7 (version / appVersion in Chart.yaml). Frontend image tags are unchanged.

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