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

bin/test

@@ -32,6 +32,13 @@ docker run --rm "$img" bash -ec '
     gs --version
     ruby --version
     bundle --version
+    valkey-server --version
+    valkey-cli --version
+    valkey-server /etc/valkey/valkey.conf
+    valkey-cli ping
+    valkey-cli shutdown nosave
+    psql --version
+    postgres --version
 '
 
 echo "== $img: runit service lifecycle"