Delete the kagane-specific cover path (#63) (#73)

Closes #63

Deletes the second way to reach a Cover. Since #62, every Site's cover bytes land in the content-addressed store at creation or on the poll, and the one public route serves them all — nothing needs the kagane proxy anymore.

## What went

- **Template-level rewrite:** `Bookmark.CoverURL()` and both templates' use of it. Cards and chrome now render `.Cover` — the wire value — and nothing else. `Bookmark.CoverSource` was dead once `CoverURL` went, so it and its `bookmarkColumns` entry are gone too.
- **Kagane-only cover route and its identifier validation:** `GET /img/kagane/{id}`, `web.CoverFetcher`, `coverIDRe`, and the whole `internal/web/cover.go`.
- **The proxy's persistence:** `store.KaganeImageID`, `GetKaganeCover`, `PutKaganeCover`, `kaganeCoverSourceURL`, `kaganeCoverRe`.
- **The kagane-shaped branch in the byte-fetch routing:** `fetchCoverBytes` no longer takes a `site` argument and no longer names a Site. The URL shape kagane's API publishes is claimed by the browser module itself — `kaganeImageURLRe` + `browserCoverURL` live in `latest/browser.go` with the rest of the per-Site knowledge — and `BrowserFetcher.Image` is now URL-driven (it validates the URL it will navigate to, same SSRF discipline as before). The no-plain-TLS-fallback rule for a claimed URL is preserved: a claimed address with no browser is an error, never a challenge-page fetch.

## What stayed (deliberately)

- `BrowserFetcher.Image` and the browser-backed acquisition path: kagane genuinely serves cover bytes behind the challenge + `cross-origin-resource-policy: same-origin`, so the sidecar remains the only fetcher for them — it just routes by URL claim now instead of by Site name.
- `fetcherFor`'s per-Site page routing (kagane/novelfull page fetches) — that is the page path, not a cover path.

## Acceptance criteria

- [x] Template-level kagane cover rewrite gone
- [x] Kagane-only cover route and its identifier validation gone
- [x] Tests removed/rewritten against the general route, guarantees kept: unstored + traversal-shaped addresses serve nothing (`TestPublicCoverRejectsUnknownAddress`), non-image content types never echoed (`TestPublicCoverNeverEchoesNonImage` — new; the store-side gate was already pinned by `TestCoverStoreAcceptsAnySourceURL`). Store reopen-persistence and filesystem content-addressing tests rewritten against `PutCover`/`GetCover`, no guarantee lost.
- [x] No Site name in a cover code path outside the acquisition module (`grep kagane backend`: store/web/templates/api are clean; remaining hits are `latest/browser.go` + `latest/sites.go`, tests, docs)
- [x] Web UI and panel render Covers for all six Sites (templates render the wire address; panel renders `b.cover` — untouched, it never had a kagane path)
- [x] `go test ./...` green

## Verification

- `go vet ./...` clean
- `go test ./...` — all packages pass (root 16.9s, latest 12.7s, store 12.7s, web 0.004s)
- `CGO_ENABLED=0 go build` produces the static binary
- Cover-path tests run verbosely: `TestPublicCoverServesStoredBytesUnauthenticated`, `TestPublicCoverRejectsUnknownAddress` (unknown/malformed/traversal/empty), `TestPublicCoverNeverEchoesNonImage`, `TestListRendersAcquiredCover`, `TestAcquireKaganeCoverThroughBrowser`, `TestRunOncePrefetchesKaganeCover`, `TestRunOnceRoutesNonKaganeCoverToPublicFetcher` all pass; the three `SMOKE_*` tests skip without the browser sidecar, as designed

Live browser verification of the "web UI and panel render Covers for all six Sites" criterion is being run separately with Playwright against real Site pages and a locally mocked backend.

Reviewed-on: #73
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/acquire.go

@@ -42,8 +42,8 @@ type Acquirer struct {
 	// Covers retrieves the cover bytes. Nil leaves the Cover blank and the
 	// chapter half working.
 	Covers CoverBytesFetcher
-	// BrowserCoverFetch retrieves kagane cover bytes through the browser
-	// sidecar. Nil leaves kagane Covers blank; nothing falls back to a plain
+	// BrowserCoverFetch retrieves browser-claimed cover bytes through the
+	// sidecar. Nil leaves those Covers blank; nothing falls back to a plain
 	// fetch, which would only ever retrieve a challenge page.
 	BrowserCoverFetch BrowserCoverFetcher
 	// Ctx cancels in-flight acquisitions at shutdown. A hook signature has
@@ -136,7 +136,7 @@ func (a *Acquirer) acquire(ctx context.Context, sr store.Series) {
 	if !ok {
 		return
 	}
-	bytes, contentType, err := fetchCoverBytes(ctx, sr.Site, cover, a.BrowserCoverFetch, a.Covers)
+	bytes, contentType, err := fetchCoverBytes(ctx, cover, a.BrowserCoverFetch, a.Covers)
 	if err != nil {
 		log.Printf("acquire %q: fetch cover %s: %v", sr.Key(), cover, err)
 		return