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/AGENTS.md

@@ -98,6 +98,18 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   (`Store.SetLatestChapter`), not a read-modify-write of the whole bookmark:
   it cannot revert read progress or move `updated_at`, so the old
   stale-re-read race is gone with the Get+Upsert flow.
+- **Covers are acquired at creation, then served from our own origin
+  (ADR-0007):** the first Bookmark of a Series fires `Store.OnSeriesCreated`,
+  which `latest.Acquirer` turns into one series-page fetch yielding both the
+  Latest Chapter and the cover URL; the bytes then go through
+  `latest.CoverBytesFetcher` into `Store.SetSeriesCover`. It runs in a
+  goroutine — the Reader's PUT must neither block on a Site nor fail with one
+  — and every failure is logged and dropped, leaving the Bookmark intact. The
+  wire's `cover` is the absolute `PUBLIC_BASE_URL + /covers/{sha256}` once
+  bytes exist and `""` before, never an address that 404s. `GET /covers/{addr}`
+  is public and uncredentialed: the userscript renders it on a Site's origin,
+  where no cookie or token of ours travels. A client-sent `cover` is decoded
+  and discarded, permanently (ADR-0004 compatibility).
 - **`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
@@ -115,6 +127,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `ALLOWED_ORIGINS` (comma list),
   `DATABASE_URL` (Postgres connection URL, required — no default),
   `COVER_DIR` (required filesystem volume for content-addressed Cover bytes),
+  `PUBLIC_BASE_URL` (required origin this deployment answers on, trailing
+  slash trimmed; every Cover URL on the wire is built from it, absolute
+  because the userscript renders on a Site's origin — ADR-0007),
   `PORT` (default `8080`), `DISCORD_CLIENT_ID`/`_CLIENT_SECRET`/`_GUILD_ID`/
   `_REDIRECT_URI` (required; Discord OAuth for the browser UI),
   `DISCORD_REQUIRED_ROLE` (optional role gate, empty by default),