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.

deploy/compose/rivet-config.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://rivet.dev/engine-config-schema.json",
+  "file_system": {
+    "path": "/data"
+  },
+  "singleplayer": false
+}