A newly bookmarked Series acquires its Cover at creation (#59) (#68)

Closes #59.

Part of spec #55, and the ticket that fixes the reported bug #47. Architecture: `docs/adr/0007-backend-hosts-cover-bytes.md`. Does not close #47 or #55.

## What changed

A Reader bookmarks a Series nobody holds yet — the exact case in #47 — and within seconds the list shows its artwork instead of a broken image. The first Bookmark to create a Series fires `Store.OnSeriesCreated` after commit, and the new `latest.Acquirer` turns that into **one** series-page fetch that yields both the Latest Chapter and the cover URL. The bytes go through the gated cover fetcher from #57 and are stored content-addressed through #56, so the wire carries an absolute URL on this deployment's own origin — never a third-party address, and never one that 404s.

### Store

- Migration `0009_series_cover_address.sql` adds `series.cover_address`. The two facts are now split: `series.cover` is the third-party source address the bytes came from (the acquisition path's dedupe key), `series.cover_address` is the SHA-256 they are stored under. An empty `cover_address` is precisely what "no Cover yet" means, which is the distinction both the API and the UI depend on.
- `SetSeriesCover` writes the address only after the bytes are on disk, so the wire can never name an object that is not there.
- `CoverWireURL` builds `PUBLIC_BASE_URL + /covers/<sha256>` for every scanned row, and returns `""` for a blank address.
- The cover columns are gone from `Upsert`'s `INSERT` and its `DO UPDATE`. A client-supplied cover cannot reach the shared Series row on any path, not just the creation path.
- `Open` now rejects a base URL that is not an absolute `http(s)` origin: `PUBLIC_BASE_URL=bookmarks.example.com` would otherwise start cleanly and emit addresses no browser can load.

### Acquisition

- `internal/latest/acquire.go`: one fetch, gated by the poller's own `fetchableSeriesURL` (a `series_url` arrives in a client-supplied PUT body, so without the gate a token-holder chooses what the server fetches from its own network position).
- Asynchronous and log-and-drop. The Bookmark, its progress and its Latest Chapter are already committed; a Site that is down or a cover that cannot be produced disturbs none of them.
- Bounded by a two-slot semaphore. A bulk sync creating N Series would otherwise fire N simultaneous requests from one IP — the traffic shape the poller's stagger exists to avoid.
- Cancelled at shutdown (shares the poller's context) and stamps `latest_checked_at`, so the poller does not refetch the same page a tick later.
- Browser-backed Sites (kagane, novelfull) are deliberately skipped: their pages only yield a Cloudflare challenge to the TLS client, so the request would be spent for nothing. They arrive in #62.

### Wire and route

- `GET /covers/{address}` serves the bytes publicly and uncredentialed with `Cache-Control: public, max-age=604800, immutable`. The address is gated by a `^[0-9a-f]{64}$` pattern and cross-checked against a pure function of itself before any filesystem read, so no request shaped like a traversal reaches disk.
- `PUT /bookmarks/{key}` still accepts a `cover` field and discards it, permanently. Rejecting it would break every installed userscript the moment this deploys, and ADR-0004's compatibility argument depends on those scripts continuing to work. The decode site says so in place of a TODO nobody intends to keep.
- `store.CoverContentType` canonicalises comix's non-standard `image/jpg` to `image/jpeg`, so one image cannot land under two spellings. This one was found by the live smoke test, not by reading.

### Config

`PUBLIC_BASE_URL` is new and required (cover URLs must go out absolute — the userscript renders them on third-party origins, where a relative path resolves against the Site). Documented in `.env.example`, `docker-compose.yml` (`:?` so compose fails too), `DEPLOY.md` and `backend/AGENTS.md`.

## Acceptance criteria

All twelve of #59's criteria are met; the checklist on the issue is ticked with the evidence.

## Verification

- `go test ./...` green (Docker-backed Postgres suite).
- Live smoke against a real backend + Postgres: bookmarking `comix:n8we-dungeons-and-crayons` produced `"cover": "http://127.0.0.1:8099/covers/8ce74d80…"` and `"latest_chapter": "Chapter 81"` within seconds of the PUT; `curl` on that address returned `200`, `Content-Type: image/jpeg`, `Cache-Control: public, max-age=604800, immutable`, and a 280x420 JPEG. That run is what surfaced the `image/jpg` content type.
- Mutation-checked the asynchrony test: removing the `go` from `Acquire` turns `TestAcquireDoesNotBlockTheWrite` red.

## Reviewed

Both axes of `/code-review` were run against this diff before commit. Their findings that were actionable here are folded in: the concurrency bound, the shutdown tie, the `PUBLIC_BASE_URL` validation, the missing `latest_checked_at` stamp, and a test that could not fail.

## Known sequencing

A kagane/novelfull Series created between this deploy and #62 has no cover source at all: the acquisition skips those Sites and `Upsert` no longer persists the userscript-scraped address. This is #59's stated boundary rather than a defect, but it is a user-visible gap on two Sites and should order #62 accordingly.

Reviewed-on: #68
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/cover_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"sync/atomic"
 	"testing"
 
@@ -106,7 +107,7 @@ func TestKaganeCoverServesPersistedBytesAfterRestart(t *testing.T) {
 		DiscordID: "cover-owner",
 		TokenHash: sha256.Sum256([]byte("cover-owner-token")),
 	}
-	first, err := store.Open(url, owner, coverDir)
+	first, err := store.Open(url, owner, coverDir, testCoverBaseURL)
 	if err != nil {
 		t.Fatalf("Open: %v", err)
 	}
@@ -118,7 +119,7 @@ func TestKaganeCoverServesPersistedBytesAfterRestart(t *testing.T) {
 		t.Fatalf("close first store: %v", err)
 	}
 
-	second, err := store.Open(url, owner, coverDir)
+	second, err := store.Open(url, owner, coverDir, testCoverBaseURL)
 	if err != nil {
 		t.Fatalf("reopen: %v", err)
 	}
@@ -224,3 +225,82 @@ func TestKaganeCoverWithoutFetcher(t *testing.T) {
 		t.Fatalf("status = %d, want 404", rr.Code)
 	}
 }
+
+// The acquired Cover is served from this deployment's own origin, to any
+// browser rendering a third-party page — no session, no credential (ADR-0007).
+func TestPublicCoverServesStoredBytesUnauthenticated(t *testing.T) {
+	const sourceURL = "https://cdn.asurascans.com/covers/solo.webp"
+	srv, st := newWebTestServer(t, testConfig())
+	if err := st.PutCover(sourceURL, []byte("\x00webp-bytes"), "image/webp"); err != nil {
+		t.Fatalf("PutCover: %v", err)
+	}
+
+	// The wire URL is what a client actually requests, so the path under test
+	// is taken from it rather than rebuilt by hand.
+	wire := st.CoverWireURL(store.CoverAddress(sourceURL))
+	path, ok := strings.CutPrefix(wire, testCoverBaseURL)
+	if !ok {
+		t.Fatalf("wire URL %q is not on the public origin %q", wire, testCoverBaseURL)
+	}
+	rr := getCover(t, srv, path, nil)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200 without any credential", rr.Code)
+	}
+	if got := rr.Body.String(); got != "\x00webp-bytes" {
+		t.Fatalf("body = %q, want the stored bytes", got)
+	}
+	if got := rr.Header().Get("Content-Type"); got != "image/webp" {
+		t.Fatalf("Content-Type = %q, want the stored one", got)
+	}
+	// Content-addressed bytes never change, so a client that has them must
+	// never need to ask again.
+	if got := rr.Header().Get("Cache-Control"); !strings.Contains(got, "immutable") {
+		t.Fatalf("Cache-Control = %q, want an immutable cache directive", got)
+	}
+}
+
+func TestPublicCoverRejectsUnknownAddress(t *testing.T) {
+	srv, _ := newWebTestServer(t, testConfig())
+	cases := map[string]string{
+		"unknown":   "/covers/" + store.CoverAddress("https://cdn.example/never-stored.jpg"),
+		"malformed": "/covers/not-an-address",
+		"traversal": "/covers/../../etc/passwd",
+		"empty":     "/covers/",
+	}
+	for name, path := range cases {
+		t.Run(name, func(t *testing.T) {
+			if rr := getCover(t, srv, path, nil); rr.Code == http.StatusOK {
+				t.Fatalf("%s: status = 200, want anything but a served body", path)
+			}
+		})
+	}
+}
+
+// The whole point of acquiring bytes is that the UI shows them: the card's
+// <img> must carry the public address, not a third-party URL and not a
+// placeholder.
+func TestListRendersAcquiredCover(t *testing.T) {
+	const sourceURL = "https://static.comix.to/039d/i/1/34/6a6742bf15736@280.jpg"
+	srv, st := newWebTestServer(t, testConfig())
+	if _, err := st.Upsert(st.OwnerID(), store.Bookmark{
+		Key: "comix:n8we", Site: "comix", SeriesID: "n8we", Title: "Dungeons and Crayons",
+		SeriesURL: "https://comix.to/title/n8we", UpdatedAt: 1000,
+	}); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	if err := st.SetSeriesCover("comix", "n8we", sourceURL, []byte("\xff\xd8jpeg"), "image/jpeg"); err != nil {
+		t.Fatalf("SetSeriesCover: %v", err)
+	}
+
+	req := httptest.NewRequest(http.MethodGet, "/ui/list", nil)
+	req.AddCookie(sessionCookie(t, st))
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	want := `src="` + testCoverBaseURL + "/covers/" + store.CoverAddress(sourceURL) + `"`
+	if !strings.Contains(rr.Body.String(), want) {
+		t.Fatalf("rendered list does not contain %s", want)
+	}
+}