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/store/sessions.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"database/sql"
+	"fmt"
 	"time"
 )
 
@@ -67,3 +68,13 @@ func (s *Store) DeleteSession(id string) error {
 	_, err := s.db.Exec(`DELETE FROM sessions WHERE id = $1`, id)
 	return err
 }
+
+// DeleteReaderSessions revokes every session one Reader holds — the owner's
+// remedy when a Reader's browser must be logged out everywhere at once. The
+// next request carrying any of those cookies finds no row and is rejected.
+func (s *Store) DeleteReaderSessions(readerID int64) error {
+	if _, err := s.db.Exec(`DELETE FROM sessions WHERE reader_id = $1`, readerID); err != nil {
+		return fmt.Errorf("delete sessions for reader %d: %w", readerID, err)
+	}
+	return nil
+}