Move index.ts to src/db directory to align with docs

[pejeio]

This PR restructures the project to move the index.ts file from src/ to src/db. This change aligns with the documentation, which states:

Create an index.ts file in the src/db directory and initialize the connection.

Reference: https://orm.drizzle.team/docs/get-started/postgresql-new#:~:text=Create%20a%20schema.ts%20file%20in%20the%20src/db%20directory%20and%20declare%20your%20table%3A

[celeron450]

See also #487. There are other steps besides the file structure that reference src/index.ts.