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/readers.html

@@ -0,0 +1,26 @@
+{{/* The owner's Reader roster. Rendered only for the owner (listView.Owner),
+     and re-rendered whole as the response to a revocation so the session
+     counts cannot describe the state before the tap. Revocation is
+     confirm-gated: it signs someone out of every device at once. */}}
+{{define "readers"}}
+<details class="setup" id="readers">
+  <summary>Readers</summary>
+  <p class="setup-copy">Everyone who has signed in through Discord. Revoking
+    signs a Reader out of every device; their library and bookmarks are
+    untouched, and they can sign in again.</p>
+  <ul class="readerlist">
+    {{range .Readers}}
+    <li>
+      <span class="reader-id">{{.DiscordID}}</span>
+      <span class="reader-sessions">{{.Sessions}} session{{if ne .Sessions 1}}s{{end}}</span>
+      {{if .Sessions}}
+      <form hx-post="/readers/{{.ID}}/revoke" hx-target="#readers" hx-swap="outerHTML"
+            hx-confirm="Revoking signs this Reader out on every device immediately. Revoke?">
+        <button type="submit" class="ghost danger">Revoke sessions</button>
+      </form>
+      {{end}}
+    </li>
+    {{end}}
+  </ul>
+</details>
+{{end}}