Add /auth/clear to reset stale cookies

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

bp/auth/routes.py

@@ -211,6 +211,15 @@ def register(url_prefix="/auth"):
         resp.delete_cookie("sso_hint", domain=".rose-ash.com", path="/")
         return resp
 
+    @auth_bp.get("/clear/")
+    async def clear():
+        """One-time migration helper: clear all session cookies."""
+        qsession.clear()
+        resp = redirect(federation_url("/"))
+        resp.delete_cookie("blog_session", domain=".rose-ash.com", path="/")
+        resp.delete_cookie("sso_hint", domain=".rose-ash.com", path="/")
+        return resp
+
     @auth_bp.get("/sso-logout/")
     async def sso_logout():
         """SSO logout: clear federation session + sso_hint, redirect to blog."""