Add hCaptcha support alongside Turnstile and reCAPTCHA

- Add hcaptcha_site_key and hcaptcha to captcha_provider options in admin settings
- Update captcha.ts server-side verification for hCaptcha (new endpoint + secret key)
- Add hCaptcha widget rendering in register-form.tsx
- All TypeScript and Biome checks pass

src/components/auth/register-form.tsx

@@ -322,6 +322,9 @@ export function RegisterForm({
 				{showCaptcha && captcha.provider === "recaptcha" && (
 					<div className="g-recaptcha" data-sitekey={captcha.siteKey} />
 				)}
+				{showCaptcha && captcha.provider === "hcaptcha" && (
+					<div className="h-captcha" data-sitekey={captcha.siteKey} />
+				)}
 
 				{/* Submit */}
 				<button