Add persistent database(s)

Currently everything is jank and in-memory. We should move to redis (for sessions) and another database (or again redis?) for keeping movies, movie lists, search results, etc. cached. Then we can have more than one process, less crashes and persistence over restarts.

No files, tests, or entry points are named. Start by locating the current in-memory sessions, movies, movie lists, and search results, then determine how persistence should support multiple processes and survive restarts; done means those data and session requirements are stored persistently with a clear database choice.

Written by the indexing model from the issue text.