chore(deps): update postgres docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change
postgres	major	15-alpine → 18-alpine

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[sentry]

Bug: Upgrading the PostgreSQL image from version 15 to 18 without a data migration strategy will cause the database service to fail on startup if an existing data volume is present.
_{Severity: CRITICAL}

Suggested Fix

Include a data migration strategy for existing deployments. This could involve adding a manual upgrade step to the deployment process using pg_dump and pg_restore, or creating an initialization script that handles the data migration automatically. Alternatively, if this is for new deployments only, document that the volume must be cleared.

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#L22

Potential issue: The `docker-compose.yml` file updates the PostgreSQL image from version
15 to 18. PostgreSQL data directories are not compatible across major versions. When the
new container starts, it will attempt to use the existing data in the persistent volume
`staryl-postgres-data`. This will cause the database service to fail to start due to the
data format incompatibility, as no migration logic (like `pg_upgrade`) or initialization
scripts to handle the version change are included.

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

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