Set Character Sharing

Enable/disable sharing for a character (owner only). Characters have no public/private flag, so this is the mint/revoke path: enabling mints a share_token (reusing any existing one so live links survive re-share); disabling NULLs it, instantly killing existing ?token= links. Returns the token so the client can build the …/share/character/{id}?token=… link. Body: {"shared": bool}