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.

.env.example

@@ -6,17 +6,10 @@
 #   openssl rand -hex 32
 TOKEN_KEY=changeme-generate-a-long-random-token
 
-# Retired global credential, kept only during the cutover window so
-# already-installed scripts keep working. Remove both it and
-# API_TOKEN_GRACE_UNTIL once the window has passed and every device has
-# reinstalled through the web UI.
-# API_TOKEN=
-# Moment the retired credential stops resolving to the owner (YYYY-MM-DD or
-# RFC3339). Enforced in code on every request; unset means it is already dead.
-# API_TOKEN_GRACE_UNTIL=2026-08-22
-
-# The owner's Discord user ID — the one Reader every bookmark belongs to
-# (seeded at startup). Discord snowflake, e.g. 1046923170000000000.
+# The owner's Discord user ID — seeded at startup as the first Reader, the
+# administrator (the only one who can revoke another Reader's sessions), and
+# the owner of every bookmark that predates registration. Discord snowflake,
+# e.g. 1046923170000000000.
 OWNER_DISCORD_ID=changeme-your-discord-user-id
 
 # Comma-separated origins allowed to call the API (CORS). Both Asura domains
@@ -42,10 +35,12 @@ POSTGRES_PASSWORD=changeme-generate-a-long-random-password
 # TRAEFIK_CERTRESOLVER=le
 
 # --- Web UI (Discord OAuth) ---
-# Sign-in is a Discord authorization code grant (ADR-0002): the owner signs in
-# with Discord, and guild membership gates access. Create the application at
-# https://discord.com/developers/applications and register the exact callback
-# URL ($BOOKMARK_WEB_HOST/auth/discord/callback) as an OAuth2 redirect.
+# Sign-in is a Discord authorization code grant (ADR-0002), and it is also
+# registration: any member of the configured guild becomes a Reader on their
+# first successful login, with their own empty library. Create the application
+# at https://discord.com/developers/applications and register the exact
+# callback URL ($BOOKMARK_WEB_HOST/auth/discord/callback) as an OAuth2
+# redirect.
 DISCORD_CLIENT_ID=
 DISCORD_CLIENT_SECRET=
 # The guild whose membership gates sign-in (Developer Mode -> right-click the