feat(01-01): Prisma schema with Tenant/User models and Vitest test setup

- prisma/schema.prisma with Tenant, User, Role, TenantStatus models
  - tenantId on all tenant-scoped models (RLS-ready convention)
  - @@unique([email, tenantId]) and @@index([tenantId]) on User
  - Grace period fields on Tenant (suspendedAt, gracePeriodEndsAt)
  - RLS comment block documenting tenantId convention for future models
- src/lib/prisma.ts singleton PrismaClient pattern (hot-reload safe)
- vitest.config.ts with node environment and @/* path alias
- src/lib/__tests__/setup.test.ts smoke test (2 tests passing)
- package.json scripts: test, test:watch, db:push, db:generate, db:studio
- Schema synced to PostgreSQL 16 via prisma db push

vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    globals: true,
+    environment: "node",
+  },
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+});