Poll comix.to through the browser sidecar (#98) (#105)

Closes #98.

comix.to began answering plain-TLS fetches with a Cloudflare JavaScript
challenge on 2026-08-12, so every poll got a 403 interstitial. Its cover host
`static.comix.to` is gated the same way. comix therefore joins kagane and
novelfull as a browser-backed Site.

## What changed

- **Registry** (`internal/latest/sites.go`): comix gains a `Browser` entry —
  `comixRead`, `Done: body != "" && !isInterstitial(body)`, `Fallback: false`.
  Skip-when-no-browser falls out of the existing routing; no site-string compare
  was added anywhere.
- **Read shape** (`internal/latest/browser.go`): an in-tab `fetch()` of the
  Series URL, not a DOM render. comix is an SPA — rendering it costs ~65
  requests for the same server-rendered HTML one fetch returns (24.5 KB,
  ~480 ms measured). `comixSeriesPageURL` pins scheme + host + `/title/<slug>`
  and rebuilds the address, so a client-supplied `series_url` cannot aim the
  browser anywhere else.
- **Cover bytes**: `comixImageURLRe` pins `https://static.comix.to/<path>.<ext>`;
  `BrowserFetcher.Image` now gates on `browserOnlyCoverURL` rather than a
  kagane-only regex, so both Sites' image URLs route through the one path.
  Bytes come from direct navigation, not a page-context fetch — comix's Series
  page sets `cross-origin-embedder-policy: require-corp`, which fails one.
- **Parsers and stored Series identity: untouched.** The in-tab body is the same
  server-rendered HTML the existing fixtures were cut from.

## Verification

- `go test ./...` green (needs Docker).
- New seam tests: comix routes to the browser when one is configured, and is
  not fetched at all when none is (`TestComixUsesBrowserFetcher`,
  `TestComixSkippedWhenNoBrowserFetcher`); URL-pin and cover-gate table tests.
- Live proof against the real browser unit, `TestSmokeComix` (env-gated):
  page 24793 bytes in one in-tab fetch, chapter 53, cover accepted by the pin,
  26862 bytes of `image/jpg` retrieved.
- Two-axis review run; findings were stale comments on `BrowserFetcher`, `Get`
  and the `Fallback` field, fixed in f000cc7.

Docs updated: root `AGENTS.md` (constraint + smoke command, including the note
that this dev machine's ISP DNS-hijacks `comix.to`), `backend/AGENTS.md`
(poller, cover pipeline, `BROWSER_WS_URL`), `REDEPLOY.md` §8 degrade note.

Reviewed-on: #105
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/sites_test.go

@@ -41,6 +41,13 @@ 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. Confirmed against a
+// live cleared tab 2026-08-16 (TestSmokeComix): the in-tab fetch returned
+// 24793 bytes of server-rendered HTML that these same parses read a chapter
+// and a cover out of.
 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"}]}