Add Valkey and PostgreSQL (closes #9)

Install valkey-server, valkey-tools and postgresql from apt. Ubuntu
26.04 ships Valkey 9 and PostgreSQL 18, replacing the postgresql16 the
audit pinned.

Model: deepseek-v4-pro

README.md

@@ -43,6 +43,8 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | Ghostscript     | apt                            | `gs`                           |
 | Ruby            | apt `ruby`, `ruby-dev`         | `/usr/bin/ruby`                |
 | Bundler         | apt `bundler`                  | `bundle` on `PATH`             |
+| Valkey          | apt `valkey-server`, `valkey-tools` | `valkey-server`, `valkey-cli` |
+| PostgreSQL      | apt `postgresql`               | `psql`, `postgres`, `initdb`, `pg_ctl` on `PATH` |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`