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/AGENTS.md

@@ -119,7 +119,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   browser-fetched page and the bytes go over plain TLS. With no browser
   configured, kagane Covers are simply absent; novelfull still gets one — at
   creation and on the poll — when its page body happens to answer a plain
-  request (the challenge is a live time-varying fact).
+  request (the challenge is a live time-varying fact). The old kagane-only
+  serving path (`/img/kagane/{id}`, template rewrite, `CoverFetcher`) is gone
+  (issue #63): the one public route serves every Site.
 - **`updated_at` drives list order, so moves only on real reading progress:** server apply its timestamp when row new or `last_chapter_num` changes, else keep stored value — favouriting series or recording newly published chapter must not reorder list. `PUT` therefore returns row **as stored**, clients must adopt that response rather than own payload. See `plans/2026-07-25-bookmark-list-favorites-design.md` §4.
 - **Lifecycle buckets:** `status` on each bookmark is `reading` | `archived` |
   `finished`, orthogonal to `favorite`. Archived and finished appear only in
@@ -156,21 +158,21 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   Reader's credential at serve time).
   `BROWSER_WS_URL` (CDP endpoint of the browser, which runs on a **separate
   machine** and is reached over the tailnet — ADR-0006, `chrome/docker-compose.yml`.
-  Used by the poller for kagane and novelfull *and* by the web UI's kagane
-  cover proxy; unset — the default — disables browser polling and serves 404
-  for covers not already stored, leaving those sites to the userscript alone.
-  Must be a tailnet IP, never a hostname: Chrome's DevTools handler 500s
+  Used by the poller for kagane and novelfull page fetches and by the cover
+  pipeline for kagane's image bytes (the browser is the only route that clears
+  the challenge kagane serves its covers behind); unset — the default —
+  disables browser polling and leaves kagane Covers blank until stored bytes
+  exist. Must be a tailnet IP, never a hostname: Chrome's DevTools handler 500s
   `/json/version` for any Host that isn't an IP or `localhost`).
-- **kagane covers are proxied, not hot-linked:** kagane serves cover images
-  behind the same challenge as its pages and with
-  `cross-origin-resource-policy: same-origin`, so no `<img>` on the web UI's
-  origin can load one — not even from a browser holding the clearance cookie
-  `og:image` to `/img/kagane/{id}`. `internal/web/cover.go` reads the persistent
-  `covers` table first, then fetches a miss through `latest.BrowserFetcher.Image`.
-  The templates render `.CoverURL`, never `.Cover`. The id is matched against a
-  UUID regex before it reaches the browser: the stored value is client-supplied,
-  so an unchecked one is an SSRF primitive pointed at the deployment's own
-  network.
+- **No per-Site cover path (issue #63):** every Cover — all six Sites — is
+  served by the one public `GET /covers/{addr}` route from content-addressed
+  bytes. There is no proxy, no per-Site rewrite, no second place that decides
+  a Cover's renderable address: the wire `cover` is it. The only place a Site
+  name still appears in cover code is the extraction module (`latest`), where
+  kagane's image URLs are claimed by `browserOnlyCoverURL` — they answer a
+  plain fetch with a challenge and `cross-origin-resource-policy: same-origin`;
+  every other Site's CDN answers plain TLS. Templates render `.Cover` — the
+  wire value — never anything else.
 - **Web UI also owns:** session-gated `GET /install/{manga,novel}-bookmark.user.js`
   (renders the bindmounted script with the acting Reader's derived credential
   substituted in — the credential never appears in page markup, the address