Move tests to __tests__ folders, add unit tests for shared utils and services

Restructure tests into __tests__/ directories at package root so they can
be excluded from production builds. Add unit tests for dates, currency,
lookup service, payment method default logic, and tax exemption state
transitions.

packages/backend/__tests__/routes/v1/health.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect, beforeAll, afterAll } from 'bun:test'
+import type { FastifyInstance } from 'fastify'
+import { createTestApp } from '../../../src/test/helpers.js'
+
+describe('GET /v1/health', () => {
+  let app: FastifyInstance
+
+  beforeAll(async () => {
+    app = await createTestApp()
+  })
+
+  afterAll(async () => {
+    await app.close()
+  })
+
+  it('returns ok when db and redis are connected', async () => {
+    const response = await app.inject({
+      method: 'GET',
+      url: '/v1/health',
+    })
+
+    expect(response.statusCode).toBe(200)
+
+    const body = response.json()
+    expect(body.status).toBe('ok')
+    expect(body.db).toBe('connected')
+    expect(body.redis).toBe('connected')
+    expect(body.timestamp).toBeDefined()
+  })
+})