fix: add django.contrib.postgres to INSTALLED_APPS for Django 6.0

Django 6.0 requires django.contrib.postgres to be explicitly added to
INSTALLED_APPS when using PostgreSQL-specific features like SearchVectorField
and GinIndex.

This fixes the following error on Heroku:
- wagtailsearch.IndexEntry: (postgres.E005) 'django.contrib.postgres' must be
  in INSTALLED_APPS in order to use SearchVectorField and GinIndex.

All 33 tests pass successfully.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: matrixise

pythonie/pythonie/settings/base.py

@@ -43,6 +43,7 @@
     "django.contrib.sessions",
     "django.contrib.messages",
     "django.contrib.staticfiles",
+    "django.contrib.postgres",
     "compressor",
     "taggit",
     "modelcluster",