chore(deps): update docker.io/library/postgres docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change
docker.io/library/postgres (source)	major	16-alpine → 18-alpine

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[Copilot]

Pull request overview

This PR updates the PostgreSQL Docker image from version 16-alpine to 18-alpine across multiple services in the infrastructure. This is a major version upgrade automatically proposed by Renovate's dependency management.

Changes:

• Upgrades PostgreSQL from version 16 to version 18 across all affected services
• Updates affect both Authentik (authentication service) and Stalwart (mail server) deployments
• Changes span multiple hosts (web1, web3, web8, mx1)

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
web8/containers/stalwart/docker-compose.yml	Updates Postgres image to v18 for Stalwart mail service on web8
web8/containers/authentik/docker-compose.yml	Updates Postgres image to v18 for Authentik service on web8
web3/containers/authentik/docker-compose.yml	Updates Postgres image to v18 for Authentik service on web3
web1/containers/authentik/docker-compose.yml	Updates Postgres image to v18 for Authentik service on web1
mx1/containers/stalwart/docker-compose.yml	Updates Postgres image to v18 for Stalwart mail service on mx1

[anatolinicolae]

This needs a manual upgrade for major versions.

[coderabbitai]

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