This is a major breaking change that moves away from having the
entire app configuration in external TOML files to settings being
in the database with a UI to update them dynamically.
The app loads all config into memory (app settings, SMTP conf)
on boot. "Hot" replacing them is complex and it's a fair tradeoff
to instead just restart the application as it is practically
instant.
A new `settings` table stores arbitrary string keys with a JSONB
value field which happens to support arbitrary types. After every
settings update, the app gracefully releases all resources
(HTTP server, DB pool, SMTP pool etc.) and restarts itself,
occupying the same PID. If there are any running campaigns, the
auto-restart doesn't happen and the user is prompted to invoke
it manually with a one-click button once all running campaigns
have been paused.
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