Refactor auxiliary backend modules into TS (config, db)

[jescalada]

Is your feature request related to a problem? Please describe.
As part of the #276 epic, we could split the TS refactor down into more manageable chunks:

• Frontend
• Backend
  • service: Web service/auth layer
  • proxy: Proxy logic
  • Auxiliary modules
    • config
    • db
• Tests

Describe the solution you'd like
This issue is only for the auxiliary modules (config and db). I don't think a total conversion is possible (due to integration issues with unconverted modules), but we can start adding type definitions for common structures in the backend.

Describe alternatives you've considered
None.

Additional context
#927

[jescalada]

The related PRs have been included and merged in #960.