fix: Add PostgreSQL support for Django 6.0 compatibility

[matrixise]

Summary

This PR fixes the Django 6.0 compatibility issue by adding proper PostgreSQL support for tests and moving psycopg to main dependencies.

Problem

After upgrading to Django 6.0, Wagtail requires django.contrib.postgres to be in INSTALLED_APPS when using PostgreSQL-specific features like SearchVectorField and GinIndex. However, adding this to INSTALLED_APPS caused CI tests to fail because:

1. django.contrib.postgres requires psycopg to be installed
2. psycopg was only in requirements/production.txt
3. CI tests only installed main.txt and dev.txt
4. Tests failed with ModuleNotFoundError: No module named 'psycopg'

Solution

1. Add PostgreSQL to CI Tests (.github/workflows/test.yml)

• Added PostgreSQL 17 service to GitHub Actions
• Configured DATABASE_URL to use PostgreSQL in CI
• Tests now run in an environment similar to production

2. Conditional Database Configuration (pythonie/pythonie/settings/tests.py)

• Tests use PostgreSQL when DATABASE_URL is set (CI)
• Tests use SQLite when DATABASE_URL is not set (local development)
• Maintains simplicity for local development while ensuring production-like testing in CI

3. Move psycopg to Main Dependencies

• Moved psycopg[binary] from requirements/production.in to requirements/main.in
• Since django.contrib.postgres is now in base INSTALLED_APPS, psycopg is a core dependency
• Recompiled all requirements files

Changes

• ✅ PostgreSQL 17 service added to GitHub Actions
• ✅ Conditional database configuration for tests
• ✅ psycopg moved to main dependencies
• ✅ All tests passing in CI

Testing

• ✅ CI tests pass with PostgreSQL: https://github.com/PythonIreland/website/actions/runs/20488057002
• ✅ Local development can still use SQLite (no PostgreSQL required locally)
• ✅ Production compatibility maintained

Related Issues

Fixes the deployment error from Django 6.0 upgrade:

wagtailsearch.IndexEntry: (postgres.E005) 'django.contrib.postgres' must be 
in INSTALLED_APPS in order to use SearchVectorField and GinIndex.

Checklist

• Tests pass in CI
• PostgreSQL compatibility verified
• Local development workflow maintained
• Requirements files updated and recompiled