Implement keyframe transitions

# Priority This is a feature that is all the rage now, so I don't think we should wait 2 months to implement this. Generally, I would say that new AI/features should be implemented in < 2 weeks time spans (à la Hugging Face and Replicate) so that we do not become too old (AI trends move fast) # Context Some AI models support keyframe transitions (there are a lot of ComfyUI workflows to do this, with animatediff & co) Some do it particularly well, for instance Luma Labs: https://x.com/LumaLabsAI/status/1835742817060331783 The good news that that Clapper isn't too far from being able to do this: this can be done by implementing a `ClapSegmentCategory.TRANSITION` resolver, that would take the shots on both ends (start and end) and gracefully merge them using the selected transition workflow (`ClapWorkflowCategory.VIDEO_TRANSITION`) # Solution - add a rule when resolving a transition to grab the shots at both ends - use the keyframing system from LumaLabs to implement it