feat(web): vercel deployment and vds compose/systemd setup

Add Vercel config with React Router preset, vercel-build script and
/api/auth rewrite to the Convex site. Add VDS staging compose for the
Rivet engine and runner plus systemd units, deployment plan, and
Docker/Vercel ignore rules. Lockfile covers both the web and agents
dependency additions.

vercel.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "buildCommand": "pnpm run vercel-build",
+  "installCommand": "pnpm install --frozen-lockfile",
+  "rewrites": [
+    {
+      "source": "/api/auth/:path*",
+      "destination": "https://joyous-cat-297.convex.site/api/auth/:path*"
+    }
+  ]
+}