Switch backend to PostgreSQL

[pganssle]

Currently I'm using some sort of cobbled-together database of YAML files. I was hoping that this would be performant enough for the short term to not make much difference, but with only around 450 books there's significant lag in initial startup, and it causes some issues.

The main advantage to using YAML files was that it would be easy to edit them by hand before I had a proper interface for manual tweaks to metadata, but I think the better approach to this is the one I took with find_missing_books, where I dump a subset of the database as YAML, then update the full database from there. I think something like that should be an easier stop-gap before I get a proper editing system.