Create database diagram

[eloquence]

We have a (currently outdated) database diagram of the SecureDrop Core database here:
https://github.com/freedomofpress/securedrop-docs/blob/main/docs/diagrams/securedrop-database.png

It would be good to create one for the client database as well, and to keep it up-to-date with schema changes, so it is easier to reason about the database.

[eloquence]

I would suggest that we include cardinalities (e.g., one-to-many) from the start (using whichever notation folks prefer).