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/web/templates/card.html

@@ -6,7 +6,7 @@
   <div class="row">
     <a class="cover" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer"
        tabindex="-1" aria-hidden="true">
-      {{if .CoverURL}}<img src="{{.CoverURL}}" alt="" loading="lazy">
+      {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
       {{/* aria-hidden on the cover link is not enough — Chromium still exposes
            the letter because the link is programmatically focusable — so the
            monogram carries its own, same as the recent strip's. */}}