Promise executor functions should not be async in DB code

[kriswest]

Describe the bug
Linting reports "Promise executor functions should not be async" in src/db/index.ts and src/db/file/repo.ts. Update their implementations to use promise chains and ensure that if an error occurs the promise rejects (which may not be the case currently where the implementation lacks try/catch clauses).

To Reproduce
Steps to reproduce the behaviour:

1. Run linting - currently via chore: update to eslint v9 #955 (which will be merged shortly)

Expected behavior
Clean report from linting