feat: add EU CAPTCHA as a first-class captcha provider

[saschaatmyra]

EU CAPTCHA (eu-captcha.eu) is a GDPR-compliant, privacy-first anti-bot service hosted entirely within the EU. This commit adds it as a native provider alongside hcaptcha, recaptcha, and turnstile.

Key differences from existing providers that required code changes:

• Verification uses a JSON body instead of form-encoding, and must include sitekey, client_ip, and client_user_agent in addition to the token and secret.
• At startup, sitekey/secret validity is checked via the dedicated /verify-credentials endpoint and the result is logged.

Changes:

• pkg/configuration/configuration.go: add EucaptchaProvider constant; include it in the validateCaptcha allowlist.
• pkg/captcha/captcha.go: register eucaptcha in infoProviders (including credentialsCheck URL); extend Validate() to accept remoteIP so client context can be forwarded; extract validateEucaptcha() for the JSON verification path; add checkEucaptchaCredentials() for startup credential probing; update ServeHTTP() call accordingly. All existing providers are unaffected.
• README.md, examples/captcha/README.md: document the new provider.

Configuration:
captchaProvider: eucaptcha
captchaSiteKey:
captchaSecretKey:
captchaGracePeriodSeconds: 1800 captchaHTMLFilePath: /captcha.html