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/poller.go

@@ -17,8 +17,8 @@ type Fetcher interface {
 }
 
 // BrowserCoverFetcher retrieves one cover's bytes through the browser-backed
-// path — the only route that clears the challenge kagane's image URLs answer
-// a plain fetch with. Satisfied by BrowserFetcher.
+// path — the only route that clears the challenge kagane's and comix's image
+// URLs answer a plain fetch with. Satisfied by BrowserFetcher.
 type BrowserCoverFetcher interface {
 	Image(ctx context.Context, imageURL string) (body []byte, contentType string, err error)
 }
@@ -119,9 +119,9 @@ func (p *Poller) storeCover(ctx context.Context, sr store.Series, sourceURL stri
 
 // fetcherFor returns the fetcher a site's page needs, or nil when the site
 // cannot be fetched at all right now. A Site whose registry entry carries a
-// Browser read — kagane and novelfull, both behind a Cloudflare JavaScript
-// challenge no TLS fingerprint clears — prefers the browser; when it is
-// absent, the entry's Fallback decides whether plain TLS may take over. One
+// Browser read — kagane, comix and novelfull, all behind a Cloudflare
+// JavaScript challenge no TLS fingerprint clears — prefers the browser; when it
+// is absent, the entry's Fallback decides whether plain TLS may take over. One
 // routing rule for the poll and the acquirer, so the two cannot drift apart.
 func fetcherFor(site string, browser, tls Fetcher) Fetcher {
 	s, known := sites[site]