feat: Add database configuration for integration tests

Add DATABASES and required Django apps to test_settings.py to support
integration tests that use real ORM operations (wallet lifecycle tests).

This change is necessary because:
- Wallet tests verify model lifecycle (PENDING → ACTIVE → ERASED)
- Tests verify ForeignKey relationships between Payment and Wallet
- Tests verify database-level behavior (refresh_from_db, transactions)
- Mock-based tests would not catch integration issues

Following the pattern from testapp/settings.py, we use:
- In-memory SQLite for speed
- django.contrib.contenttypes (required for ForeignKey)
- django.contrib.auth (common Django dependency)

This is consistent with the pre-existing test structure where testapp
provides real models for integration testing.

Author: PetrDlouhy

test_settings.py

@@ -13,4 +13,18 @@
 SECRET_KEY = "NOTREALLY"
 PAYMENT_HOST = "example.com"
 
-INSTALLED_APPS = ["payments", "django.contrib.sites"]
+INSTALLED_APPS = [
+    "payments",
+    "django.contrib.sites",
+    "django.contrib.contenttypes",
+    "django.contrib.auth",
+]
+
+# Database configuration for tests that use ORM operations
+# (e.g., wallet tests that verify model lifecycle and relationships)
+DATABASES = {
+    "default": {
+        "ENGINE": "django.db.backends.sqlite3",
+        "NAME": ":memory:",
+    }
+}