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

build.sh

@@ -31,7 +31,8 @@ apt-get -y install --no-install-recommends \
     zstd sqlite3 \
     libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc \
     imagemagick ghostscript \
-    ruby ruby-dev bundler
+    ruby ruby-dev bundler \
+    valkey-server valkey-tools postgresql
 
 # --- Go (latest) -------------------------------------------------------------
 gover="$(curl -fsSL 'https://go.dev/VERSION?m=text' | head -n1)"
@@ -118,6 +119,9 @@ magick --version
 gs --version
 ruby --version
 bundle --version
+valkey-server --version
+valkey-cli --version
+psql --version
 hugo version
 node --version
 npm --version