Watch folders for new/changed/removed photos

[SmilyOrg]

Is your feature request related to a problem? Please describe.
Actually using the app in day-to-day life is not a great experience. You have to rescan all the time, which can take forever. If you haven't rescanned in a while, the newly added photos can also take minutes to hours to finish indexing. Additionally, there are a bunch of caches that aren't invalidated when scanning, even if you refresh the page, which seems broken and is very annoying.

Describe the solution you'd like

• Watch for files in realtime, using https://github.com/fsnotify/fsnotify
• Handle adding new files
• Handle removing files
• Handle file renames
• Handle folder renames
• Never (?) delete metadata on files so that e.g. moves do not require reindexing. This is also important in case you add e.g. tags to files
• Add marking scenes as "stale" if the dependencies (i.e. files) change and refresh them, at least upon page refresh

Describe alternatives you've considered
Improving the caching part without adding watch.

Additional context
Add any other context or screenshots about the feature request here.

[ckyb]

To start, would you accept a simple -scan command via the cli?
Something like:
./photofield -scan <folder>
./photofield -scan-all

This would be similar to hitting "rescan" in the UI, but could be run periodically (or on first import of a bunch of folders).

[SmilyOrg]

Sure! That's a good idea!

Do you think it makes sense to be able to specify any folder even if it's not part of a collection?

Should you be able to specify a collection in addition/instead? (A collection can consist of multiple folders)