chore(deps): update postgres docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change
postgres	major	17.7 → 18.3

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[sentry]

Bug: Upgrading PostgreSQL to version 18 changes the default data directory. The docker-compose.yml volume mount still uses the old path, which will cause the database container to fail on startup.
_{Severity: CRITICAL}

Suggested Fix

To ensure backward compatibility with existing data volumes, explicitly set the PGDATA environment variable for the PostgreSQL service in docker-compose.yml to the old path: PGDATA: /var/lib/postgresql/data. This forces the new PostgreSQL version to use the existing data directory, preventing a startup failure.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: docker-compose.yml#L3-L6

Potential issue: The pull request upgrades PostgreSQL from version 17.5 to 18.1. This
major version upgrade changes the default `PGDATA` directory from
`/var/lib/postgresql/data` to a new version-specific path. The `docker-compose.yml` file
continues to mount the persistent volume `wiki-link-data` to the old path. According to
official PostgreSQL Docker documentation, when the new version starts and finds data
from a previous version in an unexpected location without a migration path, the
container will refuse to start. This will cause a database startup failure in any
environment with existing data, making the application unavailable.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

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