Collection iteration is extracted into a seprate IAsyncEnumerable from
the currently only place where it happens. This is in preparation for
handling paginated collections.
This commit lays the groundwork for a user-configurable image processing pipeline. It has exactly the same behavior as the old ImageProcessor, just modular & compartmentalized. It also adds support for AVIF & JXL encoding, though no code paths call it just yet.
This implementation adds handling of incoming mentions, including rewriting non-canonical mentions of split domain users into their canonical form when inserting notes into the database.
