Add Postgres migration + seed pipeline with env template

[Jakebhudson]

This PR sets up a Postgres migration + seed pipeline for the backend prototype:

Database migration script (scripts/migrate.js) and initial schema (migrations/001_init.sql)

Seeding script (scripts/seedFromJson.js) to import the mock dataset into Postgres (≈8000 rows)

Updated repositories to load .env correctly with absolute paths

Added .env.example so team members can easily configure their local environment

Updated package.json scripts:

npm run migrate → run migrations

npm run seed → seed data from JSON

npm run dev → start server

How to run locally

1. Copy .env.example to .env and update values if needed.
2. Start Postgres (brew services start postgresql@16).
3. Run migration: npm run migrate
4. Seed database: npm run seed
5. Start server: npm run dev
6. Test endpoints:
   • GET http://localhost:3000/api/streams
   • GET http://localhost:3000/api/stream-names