Open registration to guild members (#27) (#36)

Closes #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 a turned-away sign-in leaves no row
behind. `OWNER_DISCORD_ID` still seeds the owner, but only as the
administrator — it no longer gates login.

The cutover grace path goes 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. `userscript.Handler` drops its
re-derivation too — the resolved path segment is already the credential.

New surfaces: an empty library offers both install links (behind the
tab-specific empty states, so "No favourites yet" still wins), and the owner
alone gets a Readers panel with `POST /readers/{id}/revoke`. The owner's own
row is not revocable — 404, not a self-logout.

Isolation is asserted from both directions for read, modify and delete, 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.

Verified: `go test ./...` green; live smoke against a throwaway Postgres —
empty-library state in both colour branches, roster rendering, a real revoke
through the panel (target 401s next request, owner untouched), owner
self-revoke refused 404, per-Reader `/u/<cred>` and bearer auth both 200 with
404 for an unknown credential.

Reviewed-on: #36
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/templates/list.html

@@ -16,6 +16,17 @@
   <div class="empty"><strong>Nothing archived.</strong><p>Shelve a series to park it here — it keeps getting checked for new chapters.</p></div>
 {{else if eq .Tab "finished"}}
   <div class="empty"><strong>Nothing finished yet.</strong><p>Mark a series finished and it moves out of your reading list.</p></div>
+{{else if .EmptyLibrary}}
+  {{/* Nothing in either library, so the links are the only thing this page can
+       usefully say. Both scripts: the two libraries are separate installs. */}}
+  <div class="empty">
+    <strong>Nothing here yet.</strong>
+    <p>Install the userscripts, then open a series and read a chapter — bookmarks arrive on their own.</p>
+    <p class="setup-links">
+      <a class="ghost" href="/install/manga-bookmark.user.js">Install Manga script</a>
+      <a class="ghost" href="/install/novel-bookmark.user.js">Install Novels script</a>
+    </p>
+  </div>
 {{else}}
   <div class="empty"><strong>Nothing here yet.</strong><p>Bookmarks appear once the userscript records a chapter.</p></div>
 {{end}}