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

src/lib/__tests__/setup.test.ts

@@ -0,0 +1,20 @@
+/**
+ * Smoke test: verifies that the Prisma client singleton is importable
+ * and defined without needing an active database connection.
+ *
+ * This test validates the import chain:
+ *   src/lib/prisma.ts -> @prisma/client -> generated Prisma client
+ */
+import { prisma } from "@/lib/prisma";
+
+describe("Prisma client singleton", () => {
+  it("should be defined and importable", () => {
+    expect(prisma).toBeDefined();
+  });
+
+  it("should be a PrismaClient instance", () => {
+    // PrismaClient instances have $connect and $disconnect methods
+    expect(typeof prisma.$connect).toBe("function");
+    expect(typeof prisma.$disconnect).toBe("function");
+  });
+});

src/lib/prisma.ts

@@ -0,0 +1,25 @@
+import { PrismaClient } from "@prisma/client";
+
+// Singleton pattern to prevent multiple PrismaClient instances during Next.js
+// hot-module replacement in development. Without this, each hot reload creates
+// a new PrismaClient and exhausts the PostgreSQL connection pool.
+//
+// In production, module caching ensures a single instance per process.
+
+declare global {
+  // eslint-disable-next-line no-var
+  var __prisma: PrismaClient | undefined;
+}
+
+export const prisma =
+  globalThis.__prisma ??
+  new PrismaClient({
+    log:
+      process.env.NODE_ENV === "development"
+        ? ["query", "error", "warn"]
+        : ["error"],
+  });
+
+if (process.env.NODE_ENV !== "production") {
+  globalThis.__prisma = prisma;
+}