Load API Key and Database Connection Details from `.env`

[marcbachan]

Describe the bug
We are currently hardcoding the database connection details for Supabase in src/services/db.ts. We should use an .env file instead.

Expected behavior
We should have an example.env in the repository with anonymized or null keys, and developers should create their own .env from it for local development. API keys should be stored as secrets and shared securely by some other means.