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/cover.go

@@ -22,22 +22,19 @@ type CoverBytesFetcher interface {
 	Fetch(ctx context.Context, sourceURL string) (body []byte, contentType string, err error)
 }
 
-// fetchCoverBytes routes a cover's byte retrieval by Site: only kagane needs
-// the browser for image bytes — its covers answer a plain fetch with a
-// challenge and `cross-origin-resource-policy: same-origin` — while every
-// other Site's CDN answers plain TLS. Missing fetchers degrade to an error the
-// caller logs, never a fallback onto a path that cannot succeed. One routing
-// rule for the poll and the acquirer, so the two cannot drift apart.
-func fetchCoverBytes(ctx context.Context, site, cover string, browser BrowserCoverFetcher, tls CoverBytesFetcher) ([]byte, string, error) {
-	if site == "kagane" {
+// fetchCoverBytes routes a cover's byte retrieval by URL shape, not by Site
+// name: the browser fetcher's module claims the addresses only it can fetch
+// (kagane's image route answers a plain fetch with a challenge and
+// `cross-origin-resource-policy: same-origin`), and everything else goes over
+// plain TLS. Missing fetchers degrade to an error the caller logs, never a
+// fallback onto a path that cannot succeed. One routing rule for the poll and
+// the acquirer, so the two cannot drift apart.
+func fetchCoverBytes(ctx context.Context, cover string, browser BrowserCoverFetcher, tls CoverBytesFetcher) ([]byte, string, error) {
+	if browserOnlyCoverURL(cover) {
 		if browser == nil {
 			return nil, "", errors.New("no cover fetcher")
 		}
-		imageID, ok := store.KaganeImageID(cover)
-		if !ok {
-			return nil, "", errors.New("invalid kagane cover URL")
-		}
-		return browser.Image(ctx, imageID)
+		return browser.Image(ctx, cover)
 	}
 	if tls == nil {
 		return nil, "", errors.New("no cover fetcher")