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.

src/Shared/Captcha/AltchaService.php

@@ -0,0 +1,92 @@
+<?php
+
+namespace App\Shared\Captcha;
+
+use AltchaOrg\Altcha\V1\Altcha;
+use AltchaOrg\Altcha\V1\ChallengeOptions;
+use Psr\Cache\CacheItemPoolInterface;
+
+/**
+ * ALTCHA self-hosted proof-of-work captcha.
+ *
+ * Challenges are HMAC-signed with the server secret and carry an embedded
+ * `expires` timestamp. A solved payload is accepted at most once: its signature
+ * is burned in Redis for the remaining lifetime of the challenge, so a captured
+ * payload cannot be replayed.
+ */
+class AltchaService
+{
+    private readonly Altcha $altcha;
+
+    public function __construct(
+        private readonly string $hmacKey,
+        private readonly bool $enabled,
+        private readonly int $maxNumber,
+        private readonly int $expireSeconds,
+        private readonly CacheItemPoolInterface $altchaPool,
+    ) {
+        $this->altcha = new Altcha($this->hmacKey);
+    }
+
+    public function enabled(): bool
+    {
+        return $this->enabled;
+    }
+
+    /**
+     * Build a fresh signed challenge for the widget.
+     *
+     * @return array<string, string|int> keys: algorithm, challenge, maxnumber, salt, signature
+     */
+    public function createChallenge(): array
+    {
+        $challenge = $this->altcha->createChallenge(new ChallengeOptions(
+            maxNumber: $this->maxNumber,
+            expires: (new \DateTimeImmutable())->add(new \DateInterval('PT' . $this->expireSeconds . 'S')),
+        ));
+
+        return [
+            'algorithm' => $challenge->algorithm,
+            'challenge' => $challenge->challenge,
+            'maxnumber' => $challenge->maxNumber,
+            'salt'      => $challenge->salt,
+            'signature' => $challenge->signature,
+        ];
+    }
+
+    /**
+     * Verify a base64 solution payload sent by the client.
+     * Returns false on any invalid/expired/replayed payload.
+     */
+    public function verifySolution(string $payloadBase64): bool
+    {
+        if ($payloadBase64 === '' || !$this->altcha->verifySolution($payloadBase64, true)) {
+            return false;
+        }
+
+        return $this->consumeOnce($payloadBase64);
+    }
+
+    /**
+     * Atomically burn the challenge signature so it can be used only once.
+     * Returns false if this signature has already been consumed.
+     */
+    private function consumeOnce(string $payloadBase64): bool
+    {
+        $decoded   = json_decode((string) base64_decode($payloadBase64, true), true);
+        $signature = is_array($decoded) ? ($decoded['signature'] ?? null) : null;
+        if (!is_string($signature) || $signature === '') {
+            return false;
+        }
+
+        $item = $this->altchaPool->getItem('altcha_used_' . $signature);
+        if ($item->isHit()) {
+            return false;
+        }
+
+        $item->set(true)->expiresAfter($this->expireSeconds);
+        $this->altchaPool->save($item);
+
+        return true;
+    }
+}

src/Shared/Captcha/CaptchaController.php

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+<?php
+
+namespace App\Shared\Captcha;
+
+use App\Shared\Controller\BaseController;
+use OpenApi\Attributes as OA;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\Routing\Attribute\Route;
+
+#[OA\Tag(name: 'Captcha')]
+class CaptchaController extends BaseController
+{
+    public function __construct(private readonly AltchaService $altcha) {}
+
+    #[OA\Get(
+        path: '/api/v1/altcha/challenge',
+        summary: 'صدور یک challenge امضاشده‌ی ALTCHA برای حل proof-of-work سمت مرورگر',
+        responses: [new OA\Response(response: 200, description: 'ALTCHA challenge object')]
+    )]
+    #[Route('/api/v1/altcha/challenge', methods: ['GET'])]
+    public function challenge(): JsonResponse
+    {
+        return new JsonResponse($this->altcha->createChallenge());
+    }
+}

src/Shared/Captcha/CaptchaGuard.php

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+<?php
+
+namespace App\Shared\Captcha;
+
+use App\Shared\Constant\ErrorCodes;
+use App\Shared\Exception\AppException;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Drop-in captcha check for public endpoints.
+ *
+ * Call `assertValid($request)` at the top of any unauthenticated POST handler.
+ * No-op when ALTCHA is disabled (dev/test), so protected handlers stay testable
+ * without solving a proof-of-work.
+ */
+class CaptchaGuard
+{
+    public function __construct(private readonly AltchaService $altcha) {}
+
+    public function assertValid(Request $request): void
+    {
+        if (!$this->altcha->enabled()) {
+            return;
+        }
+
+        $data    = json_decode($request->getContent(), true);
+        $payload = is_array($data) ? (string) ($data['altcha'] ?? '') : '';
+
+        if (!$this->altcha->verifySolution($payload)) {
+            throw new AppException(ErrorCodes::ERR_CAPTCHA_001, null, 422, 'altcha');
+        }
+    }
+}