feat(latest): extract per-site covers (#58) (#67)

Closes #58

## Summary

- Add pure per-Site cover extraction beside latest-chapter parsing for all six Sites.
- Read Asura, Demonic, LightNovelWorld, and NovelFull metadata; read the Comix target detail state; read Kagane's browser-fetched `series_covers[].image_id` JSON.
- Preserve published cover URLs, percent-encode Demonic raw spaces, select Comix's smaller published `medium`, and avoid thumbnail rendition URL synthesis.
- Add live-source fixtures plus no-cover and Cloudflare challenge coverage for every Site.

## Correctness

- Scope Comix extraction to the requested series detail key, avoiding recommended posters.
- Parse Kagane's current live API shape and emit its canonical compressed image route from the published image ID; unrelated JSON fields are ignored.
- Validate Kagane image IDs against the existing UUID-shaped route constraint.
- Keep extraction pure; storage, polling, and wire integration remain outside issue #58.

## Acceptance criteria

- [x] Cover extraction exists for all six Sites in the existing latest parser module.
- [x] Each Site has a live-source fixture with source URL and date.
- [x] Comix reads the state blob, not metadata.
- [x] Demonic raw spaces are percent-encoded.
- [x] Comix returns the smaller published rendition.
- [x] No-cover pages return empty.
- [x] Cloudflare challenge pages return empty.
- [x] No thumbnail URL is synthesized by editing a published URL.
- [x] `go test ./...` passes.

## Verification

- `go test ./...`
- `go vet ./...`
- `git diff --check`

Parent issues #47 and #55 remain open as requested.

Reviewed-on: #67
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/sites_test.go

@@ -79,6 +79,119 @@ const lnwSeriesFixture = `
 <a href="https://lightnovelworld.net/overgeared-chapter-9999/">Chapter 9999</a>
 `
 
+// Trimmed from https://asurascans.com/comics/chronicles-of-the-demon-faction-f886a8af
+// (redirected to ...-00dcbf97) on 2026-08-10.
+const asuraCoverFixture = `<meta property="og:image" content="https://cdn.asurascans.com/asura-images/covers/chronicles-of-the-demon-faction.d4dcb8.webp">`
+
+// Trimmed from https://demonicscans.org/manga/Catastrophic-Necromancer on 2026-08-10.
+// The source publishes the raw space in this URL.
+const demonicCoverFixture = `<meta property="og:image" content="https://readermc.org/images/thumbnails/Catastrophic Necromancer.webp">`
+
+// Trimmed from https://comix.to/title/n8we-dungeons-and-crayons on 2026-08-10.
+// The state includes a recommended poster before the target detail object and
+// nested IDs inside that object; no og:image is present.
+const comixCoverFixture = `<script type="application/json" id="initial-data">{"queries":{"[\"manga\",\"recommended\",\"n8we\",1]":{"poster":{"medium":"https://static.comix.to/recommended@280.jpg","large":"https://static.comix.to/recommended.jpg"}},"[\"manga\",\"detail\",\"n8we\"]":{"chapters":[{"hid":"nested"}],"poster":{"medium":"https://static.comix.to/039d/i/1/34/6a6742bf15736@280.jpg","large":"https://static.comix.to/039d/i/1/34/6a6742bf15736.jpg"}}}}</script>`
+
+// Trimmed from GET https://kagane.to/api/v2/series/019fe11a-8670-7cf3-8343-0b02057d3787 on 2026-08-10.
+const kaganeCoverFixture = `{"series_covers":[{"cover_id":"019fe11a-84d1-714b-9cf4-2827f277f3c0","language":"en","volume_number":"1","chapter_number":null,"note":null,"image_id":"019fe11a-84c3-7fc3-a84b-88787374b617"}]}`
+
+// Trimmed from https://novelfull.com/reverend-insanity.html on 2026-08-10.
+const novelfullCoverFixture = `<meta name="image" content="https://novelfull.com/uploads/webp/novel/reverend-insanity-82661d911a.webp">`
+
+// Trimmed from https://lightnovelworld.net/novel/a-will-eternal/ on 2026-08-10.
+const lnwCoverFixture = `<meta content='https://lightnovelworld.net/wp-content/uploads/2026/03/a-will-eternal-1.webp' property='og:image'>`
+
+func TestCoverFrom(t *testing.T) {
+	const comixURL = "https://comix.to/title/n8we-dungeons-and-crayons"
+	tests := []struct {
+		name      string
+		site      string
+		seriesURL string
+		body      string
+		wantOK    bool
+		wantCover string
+	}{
+		{
+			name: "asura uses published metadata URL",
+			site: "asura", body: asuraCoverFixture, wantOK: true,
+			wantCover: "https://cdn.asurascans.com/asura-images/covers/chronicles-of-the-demon-faction.d4dcb8.webp",
+		},
+		{
+			name: "demonic escapes raw spaces",
+			site: "demonic", body: demonicCoverFixture, wantOK: true,
+			wantCover: "https://readermc.org/images/thumbnails/Catastrophic%20Necromancer.webp",
+		},
+		{
+			name: "comix takes target medium poster",
+			site: "comix", seriesURL: comixURL, body: comixCoverFixture, wantOK: true,
+			wantCover: "https://static.comix.to/039d/i/1/34/6a6742bf15736@280.jpg",
+		},
+		{
+			name: "kagane reads API cover image ID",
+			site: "kagane", body: kaganeCoverFixture, wantOK: true,
+			wantCover: "https://kagane.to/api/v2/image/019fe11a-84c3-7fc3-a84b-88787374b617/compressed",
+		},
+		{
+			name: "novelfull reads image metadata",
+			site: "novelfull", body: novelfullCoverFixture, wantOK: true,
+			wantCover: "https://novelfull.com/uploads/webp/novel/reverend-insanity-82661d911a.webp",
+		},
+		{
+			name: "lightnovelworld reads og image",
+			site: "lightnovelworld", body: lnwCoverFixture, wantOK: true,
+			wantCover: "https://lightnovelworld.net/wp-content/uploads/2026/03/a-will-eternal-1.webp",
+		},
+		{
+			name:      "later metadata cover survives empty match",
+			site:      "asura",
+			body:      `<meta property="og:image" content="">` + asuraCoverFixture,
+			wantOK:    true,
+			wantCover: "https://cdn.asurascans.com/asura-images/covers/chronicles-of-the-demon-faction.d4dcb8.webp",
+		},
+		{
+			name: "page without cover is empty",
+			site: "asura", body: `<meta property="og:title" content="No Cover">`,
+		},
+		{
+			name: "unknown site is empty",
+			site: "unknown", body: asuraCoverFixture,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, ok := coverFrom(tt.site, tt.seriesURL, tt.body)
+			if ok != tt.wantOK {
+				t.Fatalf("ok = %v, want %v (got %q)", ok, tt.wantOK, got)
+			}
+			if got != tt.wantCover {
+				t.Errorf("cover = %q, want %q", got, tt.wantCover)
+			}
+		})
+	}
+}
+
+func TestCoverFromChallenge(t *testing.T) {
+	tests := []struct {
+		site      string
+		seriesURL string
+	}{
+		{"asura", "https://asurascans.com/comics/chronicles-of-the-demon-faction-f886a8af"},
+		{"demonic", "https://demonicscans.org/manga/Catastrophic-Necromancer"},
+		{"comix", "https://comix.to/title/n8we-dungeons-and-crayons"},
+		{"kagane", "https://kagane.to/series/019f84bc-9ba0-7ed9-86f5-8b905ec7c28b"},
+		{"novelfull", "https://novelfull.com/reverend-insanity.html"},
+		{"lightnovelworld", "https://lightnovelworld.net/novel/a-will-eternal/"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.site, func(t *testing.T) {
+			if got, ok := coverFrom(tt.site, tt.seriesURL, challengeFixture); ok || got != "" {
+				t.Fatalf("cover = %q, ok = %v, want empty", got, ok)
+			}
+		})
+	}
+}
+
 func TestLatestChapterFrom(t *testing.T) {
 	const asuraURL = "https://asurascans.com/comics/chronicles-of-the-demon-faction-f886a8af"
 	const demonicURL = "https://demonicscans.org/manga/Catastrophic-Necromancer"