chore(deps): update postgres docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change	Age	Confidence
postgres	major	17-alpine → 18-alpine

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Configuration

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.21%. Comparing base (6fa7ce7) to head (a29cd38).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1140   +/-   ##
=======================================
  Coverage   40.21%   40.21%           
=======================================
  Files         206      206           
  Lines       15772    15772           
  Branches     1956     1956           
=======================================
  Hits         6342     6342           
  Misses       9430     9430           

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[sentry]

Bug: The PostgreSQL major version upgrade from 17 to 18 lacks a data migration strategy, which will cause startup failures for existing deployments with persistent data.
_{Severity: HIGH}

Suggested Fix

A migration path for existing data must be provided. This can be achieved by adding an automated migration script that uses pg_upgrade or a dump/restore process. Alternatively, provide a detailed, documented manual migration procedure for users to follow. Reverting to PostgreSQL 17 is also an option until a proper migration strategy is implemented.

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: compose.yaml#L28

Potential issue: The pull request upgrades the PostgreSQL image from version 17 to 18 in
`compose.yaml`. Major PostgreSQL versions have incompatible on-disk data formats. For
existing deployments using the persistent Docker volume `tux_postgres_data`, the new
PostgreSQL 18 container will attempt to start with a PostgreSQL 17 data directory. This
will cause an immediate startup failure because the data format is incompatible. Since
the application container depends on the database, it will also fail to start, leading
to a service outage for any existing installation.

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