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.

config/services.yaml

@@ -49,6 +49,14 @@ services:
             $baseUrl: '%env(default:default_api_ir_base_url:API_IR_BASE_URL)%'
             $token:   '%env(default::API_IR_TOKEN)%'
 
+    App\Shared\Captcha\AltchaService:
+        arguments:
+            $hmacKey:       '%env(ALTCHA_HMAC_KEY)%'
+            $enabled:       '%env(bool:ALTCHA_ENABLED)%'
+            $maxNumber:     '%env(int:ALTCHA_MAX_NUMBER)%'
+            $expireSeconds: '%env(int:ALTCHA_EXPIRE_SECONDS)%'
+            $altchaPool:    '@altcha.pool'
+
     # Persist warning+ logs to the app_log table while keeping stderr output.
     App\Shared\Logging\DbLogger:
         decorates: 'logger'