[Feature] Migrate Scene Previews To Its Own Directory

[stg-annon]

Is your feature request related to a problem? Please describe.

Currently previews reside in <generated>\screenshots\<oshash>.mp4 it's not really that intuitive, previews are also directly tied to a File, not necessarily a Scene, either of which could be a preference

Describe the solution you'd like

It would be nice if previews were in a directory that was more descriptive of its contents while also utilizing the tiered folder approach that thumbnails use, for example:
<generated>\previews\00\00\<preview_id>.mp4

while refactoring this aspect of previews it might make sense to also review if we should use the files identifying hash as the name for the preview or rather an identifier that would be associated with the Scene and not the File

adding an association between the preview and a Scene or File in the database would also allow for filtering for scenes missing previews which would be a nice bonus