refactor: migrate monorepo from pnpm to Bun runtime and workspaces

- root: bun workspaces via package.json, packageManager bun@1.4.0, scripts use bun --filter
- backend: Bun.serve, bun test (vitest removed), bun --watch dev script, tsconfig types bun
- tests: migrate 6 test files from vitest to bun:test (mock.module)
- validate: replace @hono/zod-validator with hono validator + zod safeParse (fixes TS2589)
- lockfile: bunfig.toml saveTextLockfile, regenerated bun.lock (text) for turbo
- docs: AGENTS.md, README.md, stack.md updated to Bun stack

apps/backend/test/health-check.test.ts

@@ -1,11 +1,11 @@
+import { beforeEach, describe, expect, it, mock, vi } from 'bun:test';
 import { Hono } from 'hono';
-import { beforeEach, describe, expect, it, vi } from 'vitest';
 
-vi.mock('@/lib/prisma', () => ({
+mock.module('@/lib/prisma', () => ({
   default: {
-    $queryRaw: vi.fn(),
+    $queryRaw: mock(),
   },
-  getPrismaClient: vi.fn(),
+  getPrismaClient: mock(),
   UnitOfWork: class {},
 }));
 
@@ -21,7 +21,7 @@ describe('health check routes', () => {
   });
 
   it('reports ok when the database answers', async () => {
-    vi.mocked(prisma.$queryRaw).mockResolvedValue([{ '?column?': 1 }]);
+    prisma.$queryRaw.mockResolvedValue([{ '?column?': 1 }]);
 
     const res = await app.request('/health');
 
@@ -32,7 +32,7 @@ describe('health check routes', () => {
   });
 
   it('reports degraded when the database is unreachable', async () => {
-    vi.mocked(prisma.$queryRaw).mockRejectedValue(new Error('connection refused'));
+    prisma.$queryRaw.mockRejectedValue(new Error('connection refused'));
 
     const res = await app.request('/health');