Add project scaffolding and infrastructure config

pnpm monorepo with shared packages, server, and UI workspaces.
Includes Docker multi-stage build, Postgres compose, and root TypeScript/Vitest config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    projects: ["packages/db", "server", "ui"],
+  },
+});