[CI] Pin PG to 15 and add Redis

[davidcornu]

Summary of the problem

• We're currently using the postgres:latest image which doesn't match what we're running in production
• We don't have Redis running on CI which prevents us from running some Sidekiq jobs

Describe your changes

• Use the 15 tag for the postgres container
• Add a redis service

[polypixeldev]

Looks like we have Postgres 15.12 in production - SELECT version(); returns

PostgreSQL 15.12 (Ubuntu 15.12-1.pgdg24.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0, 64-bit

[davidcornu]

@polypixeldev oh! I should have known that but for some reason I was convinced it was 14 😅. Thanks!