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/internal/web/static/style.css

@@ -290,6 +290,28 @@ button { cursor: pointer; }
   letter-spacing: .04em;
 }
 
+/* ---- reader roster (owner only): same hairline panel, one row per Reader ---- */
+.readerlist { margin: 0; padding: 0; list-style: none; }
+.readerlist li {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 4px 16px;
+  min-height: 44px;
+  border-top: 1px solid var(--rule);
+}
+.readerlist form { margin: 0 0 0 auto; }
+.reader-id { font: 400 13px/1.4 var(--font-mono); color: var(--paper); }
+.reader-sessions {
+  font: 400 11px/1.4 var(--font-mono);
+  letter-spacing: .04em;
+  color: var(--mute);
+}
+/* Revocation cuts someone off, so it wears --danger. Ember stays reserved for
+   the new-chapter signal. */
+.ghost.danger { color: var(--danger); }
+.ghost.danger:hover { color: var(--danger); border-bottom-color: var(--danger); }
+
 .chrome { display: flex; flex-direction: column; }
 
 .searchbar {