fix: align forms with netlify spam docs

[rnegron]

• Use Netlify's documented netlify-honeypot form attribute (was data-netlify-honeypot) on the feedback and speaker submission forms, and give the honeypot input type="text" to match the docs.
• Make the feedback form's subject and message required, so empty submissions are no longer accepted.
• Send Content-Type: application/x-www-form-urlencoded with a string body on the speaker submission fetch, as Netlify's AJAX example does.

Verified in the built HTML: the attributes are rendered on both forms, and lint and build pass.

Not included: reCAPTCHA. The honeypot only catches bots that fill out the rendered form; bots posting directly to Netlify can omit bot-field. If spam continues after this, custom reCAPTCHA 2 (SITE_RECAPTCHA_KEY / SITE_RECAPTCHA_SECRET) is the next step.

🤖 Generated with Claude Code

[github-actions]

¡Gracias por tu interés en Fullstack Nights y por hacer este Pull Request! Pronto estaremos en contacto. / Thanks for your interest in Fullstack Nights and for opening this pull request! We'll be in touch soon.