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/poller_test.go

@@ -116,9 +116,9 @@ type fakeCoverFetcher struct {
 	err         error
 }
 
-func (f *fakeCoverFetcher) Image(_ context.Context, imageID string) ([]byte, string, error) {
+func (f *fakeCoverFetcher) Image(_ context.Context, imageURL string) ([]byte, string, error) {
 	f.mu.Lock()
-	f.calls = append(f.calls, imageID)
+	f.calls = append(f.calls, imageURL)
 	f.mu.Unlock()
 	if f.err != nil {
 		return nil, "", f.err
@@ -778,9 +778,9 @@ func TestRunOncePrefetchesKaganeCover(t *testing.T) {
 	}
 	p.runOnce(context.Background())
 
-	body, contentType, ok, err := s.GetKaganeCover("019f84bc-9ba0-7ed9-86f5-8b905ec7c28b")
+	body, contentType, ok, err := s.CoverByAddress(store.CoverAddress(coverURL))
 	if err != nil || !ok {
-		t.Fatalf("GetKaganeCover: %v found=%v", err, ok)
+		t.Fatalf("CoverByAddress: %v found=%v", err, ok)
 	}
 	if string(body) != "cover-bytes" || contentType != "image/webp" {
 		t.Fatalf("stored cover = (%q, %q), want (cover-bytes, image/webp)", body, contentType)
@@ -877,7 +877,7 @@ func TestRunOnceRejectsInvalidKaganeCover(t *testing.T) {
 	}
 	p.runOnce(context.Background())
 
-	if _, _, found, err := s.GetKaganeCover("019f84bc-9ba0-7ed9-86f5-8b905ec7c28b"); err != nil || found {
+	if _, _, found, err := s.CoverByAddress(store.CoverAddress(coverURL)); err != nil || found {
 		t.Fatalf("invalid cover persisted = %v, err %v; want missing", found, err)
 	}
 }
@@ -902,7 +902,7 @@ func TestRunOnceWithoutCoverFetcherStillPollsKagane(t *testing.T) {
 	}
 	p.runOnce(context.Background())
 
-	if _, _, found, err := s.GetKaganeCover("019f84bc-9ba0-7ed9-86f5-8b905ec7c28b"); err != nil || found {
+	if _, _, found, err := s.CoverByAddress(store.CoverAddress(coverURL)); err != nil || found {
 		t.Fatalf("cover after nil CoverFetch = found %v, err %v; want missing", found, err)
 	}
 }