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/payments.test.ts

@@ -1,14 +1,14 @@
+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: {
     payment: {
-      findMany: vi.fn(),
-      count: vi.fn(),
+      findMany: mock(),
+      count: mock(),
     },
   },
-  getPrismaClient: vi.fn(),
+  getPrismaClient: mock(),
   UnitOfWork: class {},
 }));
 
@@ -37,8 +37,8 @@ describe('payments routes', () => {
   });
 
   it('lists payments with pagination and converts Decimal amounts', async () => {
-    vi.mocked(prisma.payment.findMany).mockResolvedValue([payment]);
-    vi.mocked(prisma.payment.count).mockResolvedValue(1);
+    prisma.payment.findMany.mockResolvedValue([payment]);
+    prisma.payment.count.mockResolvedValue(1);
 
     const res = await app.request('/payments');