chore: scaffold website (Astro 6 + Starlight + Tailwind v4)

Phase A of docs/plans/20260604-adr-0042-website.md. Scaffolds the site
under website/ from the Starlight template; adds Tailwind v4 (via
@tailwindcss/vite) bridged to Starlight with @astrojs/starlight-tailwind
(src/styles/global.css + customCss). Production build is green: static
output, Pagefind search index, sharp image optimization.

Template placeholders (title, example pages, sidebar) are left for
Phase B/D. Reconciles the ADR/plan/index wording from "Astro 5" to
"Astro 6" to match the scaffolded toolchain.

website/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "rdbms-playground-website",
+  "private": true,
+  "type": "module",
+  "version": "0.0.1",
+  "scripts": {
+    "dev": "astro dev",
+    "start": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview",
+    "astro": "astro"
+  },
+  "dependencies": {
+    "@astrojs/starlight": "^0.39.3",
+    "@astrojs/starlight-tailwind": "^5.0.0",
+    "@tailwindcss/vite": "^4.3.0",
+    "astro": "^6.3.1",
+    "sharp": "^0.34.5",
+    "tailwindcss": "^4.3.0"
+  }
+}