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

package.json

@@ -6,10 +6,17 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "db:push": "prisma db push",
+    "db:generate": "prisma generate",
+    "db:studio": "prisma studio"
   },
   "dependencies": {
+    "@prisma/client": "^6.19.2",
     "next": "16.1.6",
+    "prisma": "^6.19.2",
     "react": "19.2.3",
     "react-dom": "19.2.3"
   },
@@ -18,9 +25,11 @@
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "@vitejs/plugin-react": "^4.7.0",
     "eslint": "^9",
     "eslint-config-next": "16.1.6",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.0.18"
   }
 }