Update local dev environment to postgres 18

[cdrini]

Part of #12199 .

Note this will require everyone to run:

docker compose down
docker volume rm -f openlibrary_ol-postgres
docker compose up -d

To reset the local database.

Technical

Testing

Locally works!

Screenshot

Stakeholders

@mekarpeles @jimchamp

[Copilot]

Pull request overview

Updates the local Docker Compose override to run the dev database on a newer PostgreSQL version as part of the ongoing PostgreSQL 18 migration work.

Changes:

• Bump the default db service image from postgres:9.3 to postgres:18.3.
• Change the ol-postgres volume mount path for the db service.