fix: replace hardcoded Postgres config in examples with environment variables

[PaulGrimshaw]

Problem

Both example applications hardcode Postgres connection details in index.ts:

const postgresConfig = {
    host: "localhost",
    port: 5432,
    user: "postgres",
    password: "postgres",
    database: "dcb_test_1"
}

This doesn't work out of the box for anyone with a different local setup and is a bad pattern to model in example code.

Fix

• Read connection details from environment variables (or a DATABASE_URL connection string)
• Add a .env.example file to each example with documented defaults
• Add .env to .gitignore
• Update the example README (or root README) to explain how to configure the examples