The Goldilocks Zone is indeed a rough approximation of the region in which a planet may be habitable. It essentially evaluates the stellar energy output in the context of a planet like Earth -- how close or far could you move the Earth within that system in order to still maintain habitable temperatures. The Planetary Habitability Laboratory at the University of Puerto Rico (at Arecibo) has a great graphic to put some visuals (I know this is tangential to your question, I'm getting there...):
If you go to the webpage from where I sourced this image you will see a chart listing various habitable planet candidates. It is listed according to the "Earth Similarity Index", which accounts for stellar flux as well as planet mass and radius. The reason it is listed in that order is directly linked to your question: the Goldilocks Zone is really looking at where an Earth-like planet could be habitable. So they are accounting for planet size, and you can also see in the definition of the equilibrium temperature that they admit that exoplanet atmospheres are unknown and thus an Earth-like atmosphere is assumed.
The RationalWiki article on astrobiology points out another important factor: icy moons like Enceladus and Europa that are heated by tidal friction are also considered to be in the Goldilocks Zone. Therefore, the Goldilocks Zone could be considered to have extra "pockets" around places such as those moons. Arguably, ozone layers and magnetospheres are also considered when evaluating habitability -- as more information about the planet is gathered and processed, every new clue is added to the analysis.
It is also possible that planets inside the Goldilocks Zone may be inhabitable. For example, tidal locking may cause wild climate effects... although recent work suggests that may be less of a problem.
Onto your next point about radiation resistance. Welcome to the field of astrobiology! Studying life that can thrive in extreme conditions is essentially all that an astrobiologist does. You mentioned radiation resistant bacteria, there is also radiation resistance fungus which had been discovered thriving around the Chernobyl disaster site, and the tardigrade is an example of an actual animal that can withstand around 1000 times the amount of radiation as other animals.
Unfortunately I cannot quote a number for a maximum UV or radiation dose that DNA in general can withstand. However, different organisms have varying degrees of resistance to UV based on their respective evolutionary history, so I expect it would be inappropriate to suggest that the maximum dose that DNA alone can withstand corresponds to the maximum dose that the organism can withstand.
In my opinion (caution, I'm not an astrobiologist by training), the rate at which new discoveries are being made regarding extreme survivability of life on Earth suggests that something as simple as a minimum/maximum distance from a particular star is insufficient. So I think your question itself is part of the answer, it is an ongoing debate.