Feat: Move from Pages to a Worker and serve popular images as static files

Static assets don't count toward the free plan's 100,000 requests a day, so a
daily GitHub Action promotes the most viewed images (by Cloudflare analytics)
from R2 into public/ before deploying, and reports to ntfy. Zone rules fold
the site and ?i= into the path, since static files ignore query strings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/_headers

@@ -0,0 +1,14 @@
+# Promoted images (scripts/promote.js) have no file extension, so they'd go out without a Content-Type. Browsers detect
+# JPEG, PNG, GIF and WebP from the bytes whatever type is declared, so promote.js promotes only those.
+/web/*
+  Content-Type: image/jpeg
+  Cache-Control: public, max-age=604800
+  Access-Control-Allow-Origin: *
+/free/*
+  Content-Type: image/jpeg
+  Cache-Control: public, max-age=604800
+  Access-Control-Allow-Origin: *
+/ai/*
+  Content-Type: image/jpeg
+  Cache-Control: public, max-age=604800
+  Access-Control-Allow-Origin: *