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/browser_test.go

@@ -61,6 +61,62 @@ func TestNovelfullSeriesURL(t *testing.T) {
 		})
 	}
 }
+
+func TestComixSeriesPageURL(t *testing.T) {
+	const series = "https://comix.to/title/n8we-dungeons-and-crayons"
+	cases := []struct {
+		name string
+		url  string
+		want string
+	}{
+		{"series page", series, series},
+		{"trailing slash kept", series + "/", series + "/"},
+		// Query and fragment are dropped: only the pinned path travels.
+		{"query dropped", series + "?tab=chapters", series},
+		{"foreign host", "https://evil.example/title/x", ""},
+		{"lookalike host", "https://comix.to.evil.example/title/x", ""},
+		{"not https", "http://comix.to/title/x", ""},
+		{"not a series path", "https://comix.to/search", ""},
+		{"chapter page", series + "/11139891-chapter-80", ""},
+		{"garbage", "://nope", ""},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, ok := comixSeriesPageURL(tc.url)
+			if ok != (tc.want != "") || got != tc.want {
+				t.Fatalf("comixSeriesPageURL(%q) = %q, %v; want %q", tc.url, got, ok, tc.want)
+			}
+		})
+	}
+}
+
+// The browser is an SSRF primitive and a cover address can originate in a
+// client-supplied PUT body, so this gate decides what it may navigate to.
+func TestBrowserOnlyCoverURL(t *testing.T) {
+	cases := []struct {
+		url  string
+		want bool
+	}{
+		{"https://static.comix.to/039d/i/1/34/6a6742bf15736@280.jpg", true},
+		{"https://kagane.to/api/v2/image/019fe11a-84c3-7fc3-a84b-88787374b617/compressed", true},
+		// Every other Site's CDN answers plain TLS.
+		{"https://gg.asuracomic.net/covers/x.webp", false},
+		{"http://static.comix.to/039d/x.jpg", false},
+		{"https://static.comix.to.evil.example/039d/x.jpg", false},
+		{"https://evil.example/static.comix.to/x.jpg", false},
+		{"https://static.comix.to/039d/x.jpg?next=http://169.254.169.254/", false},
+		{"https://static.comix.to/039d/x.svg", false},
+		{"https://static.comix.to/../etc/passwd.jpg", false},
+		{"https://static.comix.to/", false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.url, func(t *testing.T) {
+			if got := browserOnlyCoverURL(tc.url); got != tc.want {
+				t.Fatalf("browserOnlyCoverURL(%q) = %v, want %v", tc.url, got, tc.want)
+			}
+		})
+	}
+}
 func TestClassifyBrowserInterruption(t *testing.T) {
 	if err := classifyBrowserError(context.Background(), true, context.Canceled); !errors.Is(err, errBrowserInterrupted) {
 		t.Fatalf("classifyBrowserError(context.Canceled) = %v, want browser interruption", err)