chore: configure Dokploy Railpack deployment

Railpack's Node provider builds this Astro project as a static site with zero
config (bun install -> bun run build -> Caddy serving dist), verified with
`railpack plan`:

- railpack.json pins provider=node explicitly, since the repo has both a
  package.json and a public/ dir, and sets TZ=UTC
- Staticfile sets root=dist and index_fallback=false. Railpack's default SPA
  fallback appends /index.html to try_files, which answers unknown URLs with the
  homepage and HTTP 200; disabling it yields a real 404 for this multi-page site
- src/pages/404.astro gives the static build a dist/404.html to serve
- build script now runs `astro check && astro build`, so a type error fails the
  deploy instead of shipping

Dokploy side: build type Railpack, container port 80 (the generated Caddyfile
listens on {$PORT:80}), and GET /health returns 200 for healthchecks.

railpack.json

@@ -0,0 +1,9 @@
+{
+    "$schema": "https://schema.railpack.com",
+    "provider": "node",
+    "deploy": {
+        "variables": {
+            "TZ": "UTC"
+        }
+    }
+}