[Feature] Add ability to define Clips/Sub-Scenes from Scenes

[UnwantedBunion]

Hi,

First, thanks to everyone who's worked on Stash, really appreciate your efforts.

I'm aware this is a near-duplicate of a couple of other feature requests, and there's a lot of overlap with several others. Please feel free to tag/move/etc. as appropriate, just thought I'd add my specific use cases.

Is your feature request related to a problem? Please describe.
Summary:
Remove unwanted parts/create "best bits" collection.

Detail:
Even for my favourite scenes, there are parts I'm not interested in. Either it's filler, or there are things going on I'd rather not watch. Having to fast-forward through a scene (or manually skip to the next marker) every time is a bit laborious. Being able to define the parts I do want, and automatically play only those, would be great.

Also, it'd be great to be able to put together a selection of "best" clips collection without having to manually process the file with ffmpeg etc.

Describe the solution you'd like
I'd propose adding the ability to define parts of Scenes as Clips/Sub-Scenes/whatever's the best name. This would involve being able to define a start and end time for the Clip. I'd imagine adding an end time to Markers would be the cleanest way to do thjs.

If there was a major barrier/opposition to extending Markers to include an end time, adding Clips to Scenes as a separate entity might be the way to go.

Describe alternatives you've considered
I've considered using Markers for this purpose, but they don't currently quite do 100% of what I'd like. There's no end time, so I can't define clips of arbitrary length. Sometimes the desired clip might be 5 seconds, sometimes it might be 20 mins.

Additional context
If they were to be implemented as a separate entity, I'd ideally like:

• The ability to add tags to a Clip (similar to Markers)
• A tab for Clips on the Scene page (similar to Markers)
• A separate Clips page to show all defined Clips (similar to Markers)

Regardless of implementation, to make this as useful as possible I'd like:

• The ability to, when playing a Scene, to only play the Clips for that scene rather than the whole lot. Have a toggleable setting that makes this the default behaviour. If this could include transitions (e.g. fade in/out) between Clips, that'd be perfect.
• The ability to download Clips as video files.

Similar feature requests

It's basically the same as these feature requests, with a bit more context:

[Feature] Optional end for markers to use them as clips
#3147

[Feature] End time for markers and a possibility to combine those subscenes to playlists
#2647

Other feature requests I could find that might use similar functionality:

[Feature] Support multiple Scenes in a single file
#3530

[Feature] auto-skip
#4168

[Feature] Playlist for markers like for scenes
#3923

[Feature] Include short "Scenes" (pre-clipped files) in the Marker browser by tag
#3917

Rate clips
#3197

[Feature] Create virtual files for compilations
#2511

Cheers!

[WithoutPants]

Related to #3530