AI transcription for videos added to steps during document updates

Description: Currently AI transcription only runs on videos uploaded during the initial creation of a Knowby. Videos added in subsequent updates do not get processed by AI.

Proposed behaviour: AI transcription should run on any video added to a Knowby regardless of whether it is during initial creation or a later update, generating steps and notes from the new video in the same way as the initial creation flow.

Use case: Creators frequently refine and update their Knowbys over time, adding new or replacement videos. Limiting AI processing to the initial creation means the feature loses its value the moment a document is updated, which undermines the efficiency gains AI is meant to provide.