feat(latest): poll comix through the browser sidecar (#98)

comix.to began answering plain-TLS fetches with a Cloudflare JavaScript
challenge on 2026-08-12, so every poll got a 403 interstitial and its cover
host static.comix.to is gated the same way. comix joins kagane and novelfull
as a browser Site: one registry entry, no plain-TLS fallback, and cover bytes
routed through the browser's image path behind a fully pinned URL pattern.

The read is an in-tab fetch of the Series URL, not a DOM render: comix is an
SPA, so rendering costs ~65 requests for the same server-rendered HTML one
fetch returns (24.5 KB, ~480 ms). Parsers and stored Series identity are
untouched.

Verified live against the real browser unit: page 24793 bytes in one fetch,
chapter 53, cover accepted by the pin and 26862 image bytes retrieved by
direct navigation (comix's Series page sets cross-origin-embedder-policy:
require-corp, so an in-page fetch of the cover host cannot work).

backend/internal/latest/sites_test.go

@@ -41,6 +41,10 @@ const challengeFixture = `<!DOCTYPE html><html><head><title>Just a moment...</ti
 // https://comix.to/title/n8we-dungeons-and-crayons fetched 2026-08-03. comix is
 // an SPA: the page ships a JSON state blob rather than a list of chapter
 // anchors, and latestChapterUrl is where the newest chapter actually lives.
+//
+// Still the right fixture after comix moved behind the challenge (#98): the
+// browser read is an in-tab fetch of the Series URL, so the body a poll parses
+// is this same server-rendered HTML, not a rendered DOM.
 const comixSeriesFixture = `
 {"firstChapterUrl":"/title/n8we-dungeons-and-crayons/5038739-chapter-1","latestChapterUrl":"/title/n8we-dungeons-and-crayons/11139891-chapter-80"},
 {""manga","recommended","n8we",1]":{"items":[{"latestChapterUrl":"/title/qqwrm-full-time-awakening/99999999-chapter-999"}]}