Foreign keys are not respected on row deletion

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

STR (in the dev env):

1. Create a user
2. Open a private window, submit a message
3. Delete that user via sqlite3 CLI with delete from users;
4. Create a new user
5. Login and look at inbox

Expected result: inbox is empty.

Actual result: inbox shows previous message

Why? Step 3 allows deleting rows from the users table when the foreign keys should prevent that (or should cascade to delete related rows).

Describe the solution you'd like

Within SQLite, we can enable the foreign_key_check pragma.

Ideally we should just switch to Postgres across the board as per #557

Additional context

This only affects the dev and test environments.