Fixed #32770 -- Added system check to ensure django.contrib.postgres is installed when using its features.

Added postgres.E005 to validate 'django.contrib.postgres' is in INSTALLED_APPS
when using:
* PostgreSQL-specific fields (ArrayField, HStoreField, range fields, SearchVectorField),
* PostgreSQL indexes (PostgresIndex and all subclasses), and
* ExclusionConstraint

The check provides immediate feedback during system checks rather than failing
later with obscure runtime and database errors.

Thanks to Simon Charette and Sarah Boyce for reviews.

tests/invalid_models_tests/test_deprecated_fields.py

@@ -3,7 +3,7 @@ from unittest import skipUnless
 from django.core import checks
 from django.db import connection, models
 from django.test import SimpleTestCase
-from django.test.utils import isolate_apps
+from django.test.utils import isolate_apps, modify_settings
 
 
 @isolate_apps("invalid_models_tests")
@@ -87,6 +87,7 @@ class DeprecatedFieldsTests(SimpleTestCase):
         )
 
     @skipUnless(connection.vendor == "postgresql", "PostgreSQL specific SQL")
+    @modify_settings(INSTALLED_APPS={"append": "django.contrib.postgres"})
     def test_postgres_ci_fields_deprecated(self):
         from django.contrib.postgres.fields import (
             ArrayField,