feat: register any guild member as a Reader (#27)

Guild membership is now the whole gate: discordCallback checks membership
(and DISCORD_REQUIRED_ROLE when set), then Store.EnsureReader creates the
Reader on first sight and returns the same row on every later login. The
refusal returns before EnsureReader, so nothing is created as a side
effect of being turned away. OWNER_DISCORD_ID keeps seeding the owner, but
only as the administrator — it no longer gates sign-in.

The cutover grace path is gone with it: API_TOKEN, API_TOKEN_GRACE_UNTIL
and the legacy branch in httpmw.ResolveReader are deleted, so a credential
authenticates exactly one Reader or nothing. That also lets
userscript.Handler drop the re-derivation — the resolved path segment is
already the credential to substitute.

New surfaces: an empty library offers both install links instead of
describing a filter (listView.Fresh, which also hides the action key it has
nothing to name), and the owner alone gets a Readers panel with
POST /readers/{id}/revoke (404 for anyone else) to sign a Reader out
everywhere.

Isolation is asserted from both directions rather than by counting one
Reader's rows, and the shared-series invariant is pinned: two Readers on
one series produce one series row, two independent progresses, one poll
per due cycle, and one Reader's delete leaves the other's bookmark and the
poll intact.

backend/AGENTS.md

@@ -21,9 +21,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   container per test binary (`TestMain` -> `pgtest.Main`) and hands each test
   its own database (`pgtest.URL(t)`). A package whose tests touch the store
   must have that `TestMain`.
-- **Single-owner store, four tables.** `readers` is keyed by Discord user ID
+- **Reader-owned store, four tables.** `readers` is keyed by Discord user ID
   and carries the SHA-256 of the Reader's userscript credential plus a
-  `token_epoch` (issue #24). Credentials are derived, never stored: `token.Token(TOKEN_KEY, discord_id, epoch)` (HMAC, `internal/token`), and only its SHA-256 sits in `readers.token_sha256`, so install URLs can be rebuilt after any restart while a database leak yields nothing but hashes. The seed creates exactly one row at startup; its epoch-0 hash is refreshed on every start **only while the row has never been rotated**, so a restart can never resurrect a rotated-away credential. Rotation is `Store.RotateToken` (epoch bump + hash rewrite in one transaction), driven by the web UI.
+  `token_epoch` (issue #24). Credentials are derived, never stored: `token.Token(TOKEN_KEY, discord_id, epoch)` (HMAC, `internal/token`), and only its SHA-256 sits in `readers.token_sha256`, so install URLs can be rebuilt after any restart while a database leak yields nothing but hashes. The seed creates the **owner** row at startup; its epoch-0 hash is refreshed on every start **only while the row has never been rotated**, so a restart can never resurrect a rotated-away credential. Every other row is created by that Reader's own first login (`Store.EnsureReader`, idempotent on `discord_id`, and it never rewrites an existing row's hash). Rotation is `Store.RotateToken` (epoch bump + hash rewrite in one transaction), driven by the web UI.
   `series` keyed `(site, series_id)`
   (`asura`|`demonic`|`comix`|`kagane`|`novelfull`|`lightnovelworld`) owns the
   shared facts — title, cover, canonical URL, `kind` (`manga`|`novel`),
@@ -32,10 +32,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `updated_at`. A bookmark is keyed `(reader_id, site, series_id)` — no
   surrogate id; the wire `key` is derived as `site:series_id` on read — and
   every store read/write is scoped to the reader it names. Auth resolves the
-  acting Reader from the presented credential (`httpmw.Auth`), and the
-  reader id travels in the request context; the retired global `API_TOKEN`
-  additionally resolves to the owner until `API_TOKEN_GRACE_UNTIL`, with
-  every such acceptance logged. Sync **last-write-wins**; the wire format
+  acting Reader from the presented credential (`httpmw.Auth`) and nothing
+  else — there is no unauthenticated-by-Reader route and no global token; the
+  reader id travels in the request context. Sync **last-write-wins**; the wire format
   stays flat (ADR-0004). `Store.Upsert` decomposes one flat body across two
   tables and enforces the ownership rule: client `title`/`series_url`/`cover`
   are written only when the series row is new (ADR-0003).
@@ -49,8 +48,15 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `internal/` tree, not just `*.go`. Sessions are rows in the `sessions`
   table: the cookie carries only an opaque id, looked up (and expiry-
   checked) on every request, and deleting the row revokes the session.
-  The owner's Discord ID is the only identity that can sign in while
-  registration is closed. UI mutations read-modify-write
+  Guild membership *is* registration (issue #27): `discordCallback` gates on
+  membership (and `DISCORD_REQUIRED_ROLE` when set) and then calls
+  `Store.EnsureReader`, so a refusal creates nothing and a returning Reader
+  reuses their row. The owner is the only Reader with administrative reach:
+  `POST /readers/{id}/revoke` (404 for anyone else) drops that Reader's
+  sessions, and the `readers` panel renders only on the owner's page.
+  A Reader with no bookmarks at all sees `listView.Fresh`, whose empty state
+  offers both install links instead of describing a filter.
+  UI mutations read-modify-write
   through `Store.Get` + `Store.Upsert` so `updated_at` rule stays one
   place. See `docs/superpowers/specs/2026-07-25-web-ui-design.md`.
   **Design-tool caveat:** templates link `/static/style.css` root-absolutely
@@ -104,10 +110,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   wipe bucket on every PUT from client that predates column. See
   `docs/superpowers/specs/2026-07-27-status-buckets-design.md`.
 - **Config via env:** `TOKEN_KEY` (derives every Reader's userscript credential;
-  required), `API_TOKEN` + `API_TOKEN_GRACE_UNTIL` (retired global credential
-  and the moment it stops resolving to the owner — both removed after the
-  cutover window, enforced in code), `OWNER_DISCORD_ID` (seeds the owner Reader;
-  required), `ALLOWED_ORIGINS` (comma list),
+  required), `OWNER_DISCORD_ID` (seeds the owner Reader — the administrator and
+  the owner of every pre-registration bookmark; required),
+  `ALLOWED_ORIGINS` (comma list),
   `DATABASE_URL` (Postgres connection URL, required — no default),
   `PORT` (default `8080`), `DISCORD_CLIENT_ID`/`_CLIENT_SECRET`/`_GUILD_ID`/
   `_REDIRECT_URI` (required; Discord OAuth for the browser UI),
@@ -132,3 +137,6 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `POST /rotate-token` (atomic epoch bump + hash
   rewrite; invalidates every installed copy, so the panel warns to reinstall
   on all devices).
+  Owner-only `POST /readers/{id}/revoke` (drops one Reader's session rows and
+  re-renders the `readers` panel; 404 for any non-owner) is the only route that
+  reaches across Readers.