Update postgres:16-alpine Docker digest to 721873c

[renovate]

Updates the shared production PostgreSQL 16 image and Runtrace backup image to the immutable 16.15 digest. The production environment override now selects the candidate; previously it silently retained the old image despite the Compose default update. The isolated Brio canary image remains unchanged.

CartHop bootstrap and Runtrace TLS tests now exercise the production image. CI also performs an isolated old-to-new data-directory upgrade, verifies data checksums, role ownership, extensions and writes, and restores the pre-upgrade logical backup with the previous runtime.

Local validation passed: CartHop unsafe-state and credential-preservation regressions, TLS policy, in-place upgrade and old-runtime logical restore, Compose configuration, and ShellCheck. Full CI is pending. This remains draft until the actual shared database backup and scoped production rollout/rollback are verified; fixture coverage is not a production backup or a guarantee for every application schema.

[renovate]

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