#310 Read/write metadata from/to disk

[jmalm]

1. Extract rating when scanning media.
2. Read metadata tags from XMP sidecar files if present (priority order: .xmp, ..xmp, .ext).
3. Write metadata to XMP sidecar whenever it is changed on a media file (currently only Rating, when the user marks / unmarks as favorite).
4. New CLI command "save_metadata" that saves the metadata for all media files (currently only Rating).
5. Add user setting in db for configuring if / how to save metadata to disk.

How to test:

1. Choose a photo in your collection that is not favorited. Copy the path on the file system.
2. Create an XMP sidecar file with a Rating=5:
   exiftool path/to/the-chosen-photo.xmp -Rating=5
3. Scan your photos (filesystem)
4. Check that the chosen photo now shows as favorited (refresh page if needed)
5. Edit "save_metadata_to_disk" in the database for your test user to "SIDECAR_FILE"
6. Mark a photo as favorite
7. Check that a .xmp file has been added with Rating=4 (or whatever your favorite_min_rating is set to)
8. Edit "save_metadata_to_disk" in the database for your test user to "MEDIA_FILE"
9. Mark another photo as favorite
10. Check that the tag has been added to the image file (if you used "MEDIA_FILE")

Part of #310 .

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[derneuere]

I tested it and all the different settings worked! The code looks good to me 👍

[jmalm]

Sounds great! Any chance of having this and LibrePhotos/librephotos-frontend#26 merged? I would love to start using it in my semi-stable environment (where I pull dev images). ;)

[derneuere]

Alright 👍 Should I open up a new issue for "Add backend support for setting save_metadata_to_disk from frontend."?

[jmalm]

Ouch. That was already part of the PR. I had just forgotten to remove the note. 😬

[jmalm]

This merge fixes #335, which can be be closed.