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.go

@@ -16,9 +16,11 @@ type Fetcher interface {
 	Get(ctx context.Context, url string) (body string, status int, err error)
 }
 
-// BrowserCoverFetcher retrieves one kagane cover through the browser-backed path.
+// BrowserCoverFetcher retrieves one cover's bytes through the browser-backed
+// path — the only route that clears the challenge kagane's image URLs answer
+// a plain fetch with. Satisfied by BrowserFetcher.
 type BrowserCoverFetcher interface {
-	Image(ctx context.Context, imageID string) (body []byte, contentType string, err error)
+	Image(ctx context.Context, imageURL string) (body []byte, contentType string, err error)
 }
 
 // Poller re-checks each bookmarked series' newest published chapter on a
@@ -44,8 +46,8 @@ type Poller struct {
 	BrowserFetch Fetcher
 	// CoverFetch is optional; failures are logged and never affect the chapter poll.
 	CoverFetch BrowserCoverFetcher
-	// CoverBytesFetch is optional; it handles non-kagane sources through the same
-	// failure-isolated prefetch path.
+	// CoverBytesFetch is optional; it handles plain-TLS sources through the
+	// same failure-isolated prefetch path.
 	CoverBytesFetch CoverBytesFetcher
 	Now             func() time.Time // injected so tests can freeze it
 	Cooldown        time.Duration
@@ -81,9 +83,10 @@ func (p *Poller) fillBlankCover(ctx context.Context, sr store.Series, body strin
 
 // prefetchCover heals Series that already carry a third-party source URL but
 // no stored address — the state left by client-supplied covers before
-// acquisition moved server-side. Every Site takes the same path; only the
-// byte fetcher differs (kagane needs the browser). New blanks have no source
-// URL and go through fillBlankCover from the series page instead.
+// acquisition moved server-side. Every Site takes the same path; fetchCoverBytes
+// routes by URL shape, so browser-claimed URLs still need the sidecar. New
+// blanks have no source URL and go through fillBlankCover from the series page
+// instead.
 func (p *Poller) prefetchCover(ctx context.Context, sr store.Series) {
 	if sr.Cover == "" || sr.CoverAddress != "" {
 		return
@@ -106,7 +109,7 @@ func (p *Poller) prefetchCover(ctx context.Context, sr store.Series) {
 // failure is logged against the Series and swallowed so the chapter poll
 // cannot see it.
 func (p *Poller) storeCover(ctx context.Context, sr store.Series, sourceURL string) {
-	bytes, contentType, err := fetchCoverBytes(ctx, sr.Site, sourceURL, p.CoverFetch, p.CoverBytesFetch)
+	bytes, contentType, err := fetchCoverBytes(ctx, sourceURL, p.CoverFetch, p.CoverBytesFetch)
 	if err != nil {
 		log.Printf("latest poll %q: fetch cover %s: %v", sr.Key(), sourceURL, err)
 		return