When importing a new file, add it to existing albums if its album matches

If I've previously imported an album, and later I import a single track which was missing, that track should be automatically added to the album just like all its previous kin were.

Trace the existing file-import flow and the logic that creates or associates albums. Check how metadata is matched for previously imported tracks, then verify that importing a missing track associates it with the matching existing album without disrupting current imports.

Written by the indexing model from the issue text.