Implement ALTCHA captcha service with challenge generation and solution verification

- Added AltchaService class for managing ALTCHA captcha challenges and solutions.
- Created CaptchaController to handle API requests for generating challenges.
- Introduced CaptchaGuard for validating captcha solutions on public endpoints.
- Developed unit tests for AltchaService to ensure challenge creation and solution verification functionality.
- Implemented integration tests for the Captcha API endpoint and captcha bypass behavior when disabled.
- Added documentation for the Captcha API in the corresponding markdown file.

assets/admin/components/ui/Altcha.tsx

@@ -0,0 +1,43 @@
+import React, { useEffect, useRef } from 'react';
+import 'altcha';
+
+// ALTCHA خودمیزبان: widget با گرفتن challenge از بک‌اند، proof-of-work را در
+// پس‌زمینه‌ی مرورگر حل می‌کند و مقدار base64 حل‌شده را در event `verified` می‌دهد.
+// این مقدار باید در بدنه‌ی درخواستِ endpoint عمومی با کلید `altcha` ارسال شود.
+
+interface AltchaProps {
+  onVerified: (payload: string) => void;
+  challengeUrl?: string;
+}
+
+// altcha-widget یک custom element است؛ به JSX معرفی می‌شود (React 19: namespace زیر React.JSX).
+declare module 'react' {
+  namespace JSX {
+    interface IntrinsicElements {
+      'altcha-widget': React.DetailedHTMLProps<React.HTMLAttributes<HTMLElement>, HTMLElement> & {
+        challengeurl?: string;
+      };
+    }
+  }
+}
+
+export default function Altcha({ onVerified, challengeUrl = '/api/v1/altcha/challenge' }: AltchaProps) {
+  const ref = useRef<HTMLElement>(null);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+
+    const onStateChange = (e: Event) => {
+      const detail = (e as CustomEvent).detail as { state?: string; payload?: string };
+      if (detail?.state === 'verified' && detail.payload) {
+        onVerified(detail.payload);
+      }
+    };
+
+    el.addEventListener('statechange', onStateChange);
+    return () => el.removeEventListener('statechange', onStateChange);
+  }, [onVerified]);
+
+  return <altcha-widget ref={ref as React.Ref<HTMLElement>} challengeurl={challengeUrl} />;
+}