Fix cross-subdomain cookie: add COOKIE_DOMAIN support

Session cookie was scoped to api.quantfortune.com only.
Setting COOKIE_DOMAIN=.quantfortune.com in .env makes the
cookie shared across all subdomains so app.quantfortune.com
can authenticate against api.quantfortune.com.

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

backend/app/routers/auth.py

@@ -19,6 +19,7 @@ APP_ENV = (os.getenv("APP_ENV") or os.getenv("ENVIRONMENT") or os.getenv("FASTAP
 IS_PRODUCTION = APP_ENV in {"prod", "production"}
 COOKIE_SECURE = True if IS_PRODUCTION else os.getenv("COOKIE_SECURE", "0") == "1"
 COOKIE_SAMESITE = (os.getenv("COOKIE_SAMESITE") or "lax").lower()
+COOKIE_DOMAIN = (os.getenv("COOKIE_DOMAIN") or "").strip() or None
 if IS_PRODUCTION and not COOKIE_SECURE:
     raise RuntimeError("Secure session cookies are mandatory in production")
 
@@ -33,6 +34,7 @@ def _set_session_cookie(response: Response, session_id: str):
         max_age=SESSION_TTL_SECONDS,
         secure=COOKIE_SECURE,
         path="/",
+        domain=COOKIE_DOMAIN,
     )