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/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