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/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
+}