Dockerize doab-check for local development

[rdhyee]

Goal

Make it easy to run doab-check locally (e.g., on a laptop) using Docker Compose, so developers don't need to manually set up Django, PostgreSQL, and all dependencies.

Motivation

• Eric has repeatedly suggested this should be runnable locally
• Currently deployed on a DigitalOcean droplet with manual setup
• Local development would make it easier to test fixes (like the reliability improvements in PR Add 429 handling, HTTP timeouts, and per-record exception isolation #5)
• Docker Compose provides a reproducible, one-command setup

Deliverables

• Dockerfile for the Django app
• docker-compose.yml with app + PostgreSQL services
• Documentation in README for local setup
• Seed data or management commands to bootstrap a useful local instance

Notes

• The app uses Django + PostgreSQL + OAI-PMH harvesting
• No Redis/Celery currently — harvesting is synchronous
• Should work on macOS (Apple Silicon) and Linux