feat: add captcha generation and verification utility

server/utils/auth/captcha.ts

@@ -0,0 +1,44 @@
+import svgCaptcha from 'svg-captcha'
+
+interface CaptchaRecord {
+  text: string
+  createdAt: number
+}
+
+const captchaStore = new Map<string, CaptchaRecord>()
+const CAPTCHA_TTL = 5 * 60 * 1000
+
+function cleanupExpired(): void {
+  const now = Date.now()
+  for (const [token, record] of captchaStore) {
+    if (now - record.createdAt > CAPTCHA_TTL) {
+      captchaStore.delete(token)
+    }
+  }
+}
+
+export function generateCaptcha(): { token: string; svg: string } {
+  cleanupExpired()
+  const { text, data: svg } = svgCaptcha.create({
+    noise: 3,
+    color: true,
+    background: '#f8f9fa',
+  })
+  const token = crypto.randomUUID()
+  captchaStore.set(token, { text, createdAt: Date.now() })
+  return { token, svg }
+}
+
+export function verifyCaptcha(token: string, text: string): boolean {
+  const record = captchaStore.get(token)
+  if (!record) return false
+  if (Date.now() - record.createdAt > CAPTCHA_TTL) {
+    captchaStore.delete(token)
+    return false
+  }
+  const match = record.text.toLowerCase() === text.toLowerCase()
+  if (match) {
+    captchaStore.delete(token)
+  }
+  return match
+}