✨ Extract age rating meta for EPUB files

[aaronleopold]

This will likely be the last PR before promoting experimental to nightly. I'll let it sit to try and test out more issues, but the overview of the changes is as follows:

• The age_rating field in MediaMetadata is now populated from one of two EPUB metadata fields: typicalAgeRange and contentRating
  • If both are present, it will take the minimum age between the two. E.g. 14-17 and 13+ will resolve to 13
• Accept a few other missed fields for EPUB metadata
• The live re-fetching during scans are now opt-in via a new preference. This should help avoid overloading the client during initial scans.
• A few other miscellaneous fixes/changes, primarily to the settings routes wrt access control