Add newsletter subscription

Right now if someone likes a post, there's no way to hear from me again unless they remember to check RSS.

I want a simple email signup that actually works end-to-end:

- signup form in the footer and at the end of posts (next to ViewCounter)
- double opt-in so I don't get spam complaints
- store subscribers somewhere cheap and reliable (Resend + Supabase, or Buttondown)
- auto-send when a post goes live with isPublished: true, using the existing rss.xml logic as a trigger
- add an endpoint to handle unsubscribe without a third-party redirect

Keep it minimal for v1. No weekly digest, no segmentation. Just "new post published -> email goes out". I can add that later if anyone actually subscribes.

Related: src/pages/rss.xml.js, src/pages/api/views/[slug].ts, src/components/Footer.astro