Delete the kagane-specific cover path (#63)

The web proxy for kagane covers is dead: 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. Remove the second way to
reach a Cover:

- Bookmark.CoverURL() and the templates' use of it: templates render
  the wire value (.Cover) and nothing else.
- GET /img/kagane/{id}, web.CoverFetcher, coverIDRe: the route and its
  identifier validation are gone, and with them web/cover.go.
- store.KaganeImageID, GetKaganeCover, PutKaganeCover,
  kaganeCoverSourceURL: the proxy's persistence.
- Bookmark.CoverSource: dead once CoverURL is gone.

Acquisition keeps the browser where kagane genuinely needs it, but the
Site name leaves the routing: kaganeImageURLRe lives in browser.go with
the rest of the per-Site knowledge, browserCoverURL claims the URLs the
sidecar alone can fetch, and fetchCoverBytes routes by URL shape with
no Site argument. No plain-TLS fallback for a claimed URL — that would
only retrieve a challenge page.

Cover tests: kagane route tests removed, the general-route guarantees
they pinned kept and re-pinned — unstored and traversal-shaped
addresses serve nothing (TestPublicCoverRejectsUnknownAddress), non-image
content types are never echoed back (TestPublicCoverNeverEchoesNonImage
+ TestCoverStoreAcceptsAnySourceURL). Store content-addressing and
reopen-persistence tests rewritten against PutCover/GetCover.

No Site name remains in a cover code path outside the acquisition
module; go test ./... green.

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 browserCoverURL(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")