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/userscript/userscript.go

@@ -10,7 +10,6 @@ import (
 
 	"bookmarkmanager/backend/internal/httpmw"
 	"bookmarkmanager/backend/internal/store"
-	"bookmarkmanager/backend/internal/token"
 )
 
 // tokenPlaceholder is what the bindmounted userscript carries where the
@@ -87,26 +86,15 @@ func Render(w http.ResponseWriter, r *http.Request, path, credential string) {
 // the route exists. The same credential authenticates the API bearer header,
 // so the two are one secret with one blast radius.
 //
-// The credential substituted is the resolved Reader's derived one, not the
-// raw path segment: while the retired global token is still accepted during
-// the grace window (httpmw.ResolveReader), an already-installed script
-// polling its legacy URL is served a copy carrying the Reader's own
-// credential, so the next update poll migrates the device onto its per-Reader
-// path — the window empties itself instead of ending in a silent 401 for
-// every device that never visited the web UI.
-func Handler(s *store.Store, tokenKey []byte, legacy string, graceUntil time.Time, path string) http.HandlerFunc {
+// The path segment is the credential itself, so once it resolves it is also
+// exactly what the served copy must carry — no re-derivation needed.
+func Handler(s *store.Store, path string) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		readerID, ok := httpmw.ResolveReader(s, legacy, graceUntil, r.PathValue("token"))
-		if !ok {
+		cred := r.PathValue("token")
+		if _, ok := httpmw.ResolveReader(s, cred); !ok {
 			http.NotFound(w, r)
 			return
 		}
-		discordID, epoch, err := s.ReaderTokenInfo(readerID)
-		if err != nil {
-			log.Printf("userscript: reader %d token info: %v", readerID, err)
-			http.NotFound(w, r)
-			return
-		}
-		Render(w, r, path, token.Token(tokenKey, discordID, epoch))
+		Render(w, r, path, cred)
 	}
 }