security: httpOnly cookies, rate limiting, nginx headers, SSE sanitization

Auth / token storage:
- JWT is now set as an httpOnly Secure SameSite=Lax cookie on login
- Add POST /auth/logout endpoint that clears the cookie
- get_current_member falls back to rh_token cookie when no Authorization header
- WebSocket auth now accepts cookie (rh_token) or optional ?token= query param
- Frontend removes all localStorage JWT access; uses credentials:"include" on
  every fetch so the httpOnly cookie is sent automatically
- Replace clearToken() with logout() that calls the server logout endpoint
- Non-sensitive rh_session flag in localStorage used only for client-side routing

Rate limiting:
- Add slowapi>=0.1.9 dependency
- /auth/login limited to 10 req/min per IP
- /auth/register limited to 5 req/min per IP

Nginx security headers:
- Add X-Frame-Options, X-Content-Type-Options, Referrer-Policy,
  X-XSS-Protection, Permissions-Policy to all responses

SSE error leakage:
- songs.py nc-scan/stream no longer leaks str(exc) to clients

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/pages/InvitePage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from "react";
 import { useParams, useNavigate } from "react-router-dom";
-import { api } from "../api/client";
+import { api, isLoggedIn } from "../api/client";
 
 interface InviteInfo {
   id: string;
@@ -19,7 +19,7 @@ export function InvitePage() {
   const [accepting, setAccepting] = useState(false);
   const [done, setDone] = useState(false);
 
-  const isLoggedIn = !!localStorage.getItem("rh_token");
+  const loggedIn = isLoggedIn();
 
   useEffect(() => {
     if (!token) return;
@@ -74,7 +74,7 @@ export function InvitePage() {
               {invite.used_at && " · Already used"}
             </p>
 
-            {isLoggedIn ? (
+            {loggedIn ? (
               <button
                 onClick={accept}
                 disabled={accepting || !!invite.used_at}