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/templates/chrome.html

@@ -30,9 +30,11 @@
 {{/* The action key. The icon strip on a card is unlabelled, so one permanent
      line under the tabs names every glyph. It follows the tab rather than the
      row: the archived and finished buckets swap Archive for Restore, and a
-     finished series has no Done to offer. */}}
+     finished series has no Done to offer. A Reader with no cards at all has
+     nothing for it to name, so it hides rather than disappearing — see the
+     note above about out-of-band swaps needing their target to exist. */}}
 {{define "keyrow"}}
-<div class="keyrow" id="keyrow" aria-label="Action key"{{if .OOB}} hx-swap-oob="true"{{end}}>
+<div class="keyrow" id="keyrow" aria-label="Action key"{{if .OOB}} hx-swap-oob="true"{{end}}{{if .Fresh}} hidden{{end}}>
   <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-play"/></svg><span>Read</span></span>
   <span class="pair brass"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-star"/></svg><span>Fav</span></span>
   <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-pencil"/></svg><span>Chapter</span></span>