Update Postgres to 15.18

[sir-sigurd]

Description

Bump RDS engine_version from 15.15 to 15.18, the latest available Postgres 15.x minor in RDS.

The recent PostgreSQL security release (15.18, 16.14, 17.10, etc.) prompted a customer (via IT) to ask that we move to the most recent minor of whichever major version we run. We're on 15.x, so the target is 15.18 — this is a minor-version patch, not the 16.x major upgrade (which can wait).

Note: apply_immediately = true, so this minor-version bump applies in place and causes a brief instance restart the moment terraform apply runs — it is not deferred to the RDS maintenance window. Control timing by choosing when to run the apply.

TODO

• CHANGELOG entry
• I promise to deploy to dev stacks (including nightly) soon after PR is merged so we really have these changes tested

🤖 Generated with Claude Code

Greptile Summary

Bumps the RDS PostgreSQL engine_version from 15.15 to 15.18 in response to a recent PostgreSQL security release, accompanied by the corresponding CHANGELOG entry.

• modules/db/main.tf: Single-line change updating engine_version to \"15.18\"; all other RDS settings (apply_immediately = true, auto_minor_version_upgrade = false, backup, deletion protection, etc.) remain unchanged.
• CHANGELOG.md: Adds a [Changed] entry under [Unreleased] for this upgrade.

Confidence Score: 5/5

Safe to merge — this is a one-line minor-version bump with no schema, config, or behavioral changes beyond the PostgreSQL engine version itself.

The change is minimal and well-scoped: a single RDS engine_version string update from 15.15 to 15.18. Both are within the PostgreSQL 15 minor-version series, so no compatibility concerns apply. The apply_immediately = true behavior (pre-existing) is acknowledged in the PR description, so operators are informed about the immediate restart on apply.

No files require special attention.

Important Files Changed

Filename	Overview
modules/db/main.tf	Single-line engine_version bump from 15.15 to 15.18; all other RDS config unchanged.
CHANGELOG.md	Added [Changed] entry for the Postgres 15.18 upgrade under [Unreleased].

Sequence Diagram

sequenceDiagram
    participant Operator
    participant Terraform
    participant AWS RDS

    Operator->>Terraform: terraform apply
    Terraform->>AWS RDS: Update engine_version 15.15 → 15.18
    Note over AWS RDS: apply_immediately=true<br/>triggers immediate restart
    AWS RDS-->>Terraform: Instance restarting…
    AWS RDS-->>Terraform: Available (PostgreSQL 15.18)
    Terraform-->>Operator: Apply complete

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_{Reviews (2): Last reviewed commit: "Add CHANGELOG entry for Postgres 15.18" | Re-trigger Greptile}

[Copilot]

Pull request overview

Bumps the module-managed RDS PostgreSQL minor version to the latest 15.x patch release and records the change in the changelog.

Changes:

• Updates the hardcoded RDS engine_version from 15.15 to 15.18.
• Adds an [Unreleased] changelog entry for the Postgres 15.18 update.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
modules/db/main.tf	Updates the RDS Postgres engine version used by the DB module.
CHANGELOG.md	Documents the Postgres minor-version bump under [Unreleased].

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[sir-sigurd]

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