[Infra] Upgrade PostgreSQL 17 → 18

[icook]

Summary

PostgreSQL 18 is stable (released September 2025, 18.1 released November 2025). Upgrade from PostgreSQL 17.

Motivation

• Stay current with PostgreSQL releases for security patches and new features
• PG18 includes up to 3× I/O performance improvements and new features like virtual generated columns and uuidv7()
• May resolve gosu vulnerabilities tracked in [Infra] Update postgres base image to resolve gosu vulnerabilities #177 if upstream rebuilds with newer Go

Checklist

• Review PostgreSQL 18 release notes for breaking changes
• Check pgmq extension compatibility with PostgreSQL 18
• Update dockerfiles/Dockerfile.postgres base image to postgres:18
• Update PATH/PG_CONFIG references from 17 to 18
• Update any version references in docs
• Run full test suite with new image
• Test migrations against PostgreSQL 18

References

• PostgreSQL 18 release notes: https://www.postgresql.org/docs/18/release-18.html
• Closes investigation from [Infra] Update postgres base image to resolve gosu vulnerabilities #177