Stack housekeeping

[gmazoyer]

The compose stack moves to PostgreSQL 18 and Valkey 9. Version 18 keeps its
data in a directory named after the major version, so the volume now mounts on
/var/lib/postgresql. The README explains the dump and the restore that a major
bump needs, in a form that serves the next bump as well.

The cache gets its own container, which no deployment reached before. The
worker processes are checked with pgrep instead of ps and grep, uv
installs the dependencies and halves the builder stage, and dependabot watches the
Dockerfile and the compose images.