chore(postgres)!: upgrade to PostgreSQL 18 with dump/restore migration

[SimmerV]

Summary

Upgrades the PostgreSQL service from 16 to 18 in both compose files, and adds a
one-time migration script so existing deployments can move their data without loss.

This implements the postgres portion of #404 — which Renovate bundled with an
emsdk bump and which has sat on hold pending a migration strategy. #404 can be
re-scoped to just the emsdk update or closed.

⚠️ Breaking change — action required for existing deployments

Deployments using the bundled postgres compose service must run a one-time
migration. PostgreSQL 18's on-disk format and the postgres:18 image's data
directory layout are both incompatible with 16, so the normal update flow
(docker compose pull && up -d) will leave Postgres unable to start until
the data is migrated:

git pull
docker compose pull
bash scripts/migrate-postgres.sh   # one-time
docker compose up -d

postgres:18 refuses to start with a clear error and
leaves the 16 data untouched — but the stack is down until the migration
runs. Fresh installs and external (non-compose) Postgres are unaffected.

Notes

Two incompatibilities have to be handled:

1. On-disk format — as with every PostgreSQL major version, an 18 container
   cannot start against a 16 data directory.
2. The postgres:18 image moved its data directory. 18+ images store the
   cluster in a version-specific subdirectory and expect the volume mounted at
   /var/lib/postgresql, not /var/lib/postgresql/data
   (Change PGDATA in 18+ to /var/lib/postgresql/MAJOR/docker docker-library/postgres#1259). Both compose files are updated accordingly.

What's included

• postgres:16 → postgres:18 in docker-compose.yml and
  docker-compose-dev.yml, with the volume remounted at /var/lib/postgresql.
• scripts/migrate-postgres.sh — dumps the existing cluster with a temporary
  old-version container, recreates the volume, and restores into 18. It reads the old
  version from the volume and the target from the compose file, so it is not
  pinned to 16→18; it is layout-aware, idempotent, no-ops on fresh installs, and
  sets MSYS_NO_PATHCONV so it also runs under Git Bash on Windows.
• Docs: a new "Upgrading PostgreSQL major versions" section in POSTGRES.md, a
  note in the README update steps, and version bumps in README.md /
  CONTRIBUTING.md.

Upgrading an existing deployment

git pull
docker compose pull
bash scripts/migrate-postgres.sh
docker compose up -d

[Copilot]

Pull request overview

Upgrades the bundled Docker Compose PostgreSQL service from v16 to v18 and adds guidance + a one-time dump/restore migration path so existing deployments can move their data volume across the major-version break.

Changes:

• Bump postgres image from postgres:16 to postgres:18 in both compose files and update the volume mount path to match the new image layout.
• Add scripts/migrate-postgres.sh to dump the existing cluster, recreate the volume, and restore into the new major version.
• Document the required one-time migration workflow in POSTGRES.md and add upgrade notes/version references in README.md and CONTRIBUTING.md.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
scripts/migrate-postgres.sh	Adds an automated dump/restore workflow intended to migrate an existing named Docker volume across Postgres major versions.
docker-compose.yml	Updates Postgres image to 18 and remounts the pgdata volume at /var/lib/postgresql.
docker-compose-dev.yml	Mirrors the production compose Postgres 18 + volume mount change for dev.
POSTGRES.md	Adds a dedicated “Upgrading PostgreSQL major versions” section describing the migration procedure and safety notes.
README.md	Updates the documented Postgres version and adds an explicit note about running the migration on major bumps.
CONTRIBUTING.md	Updates contributor docs to reference PostgreSQL 18.

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