ProjectJoinEndpoint.get() was AllowAny and used ProjectMemberInviteSerializer
(fields = "__all__"), leaking the invitee's email and token to anyone who
knew the workspace slug, project ID, and invite UUID (GHSA-2r58-hgv7-635q).
Introduce ProjectMemberInvitePublicSerializer with an explicit safe field list
that excludes `email` and `token`, and swap it in for the public GET endpoint.
The full serializer is retained for authenticated admin viewsets.
Co-authored-by: Plane AI <noreply@plane.so>
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