chore: final Phase 2 cleanup — remove stale references and unused imports

Remove unused JSONResponse import from starlette.responses in main.py.
All other stale references (CORSMiddleware, federation_tokens, X-Muxplex-Token,
window.opener, etc.) verified absent. Federation proxy endpoints confirmed present.

muxplex/main.py

@@ -31,7 +31,7 @@ from fastapi import FastAPI, Form, HTTPException, Request, WebSocket
 from fastapi.responses import HTMLResponse
 from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel, field_validator
-from starlette.responses import JSONResponse, RedirectResponse
+from starlette.responses import RedirectResponse
 
 from muxplex.auth import (
     AuthMiddleware,