Add needs_price_update field to Purchase model

Replace fragile price change detection in Purchase.save() with a
lazy dirty flag approach. A pre_save/post_save signal pair detects
price/currency changes without extra DB queries, and convert_prices()
uses the flag to determine which purchases need conversion.

- Add needs_price_update BooleanField with db_index
- Add pre_save signal to store old price/currency values
- Add post_save signal to set needs_price_update=True when price/currency changes
- Simplify Purchase.save() to remove DB reload + comparison logic
- Remove price_or_currency_differ_from() method
- Update convert_prices() to filter on needs_price_update flag
- Extract _get_exchange_rate() and _save_converted_price() helpers
- Add tests for the new behavior

games/migrations/0016_add_needs_price_update.py

@@ -0,0 +1,22 @@
+# Generated by Django 6.0.1 on 2026-05-12 11:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('games', '0015_alter_purchase_date_purchased_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='purchase',
+            name='needs_price_update',
+            field=models.BooleanField(db_index=True, default=True),
+        ),
+        migrations.RunSQL(
+            "UPDATE games_purchase SET needs_price_update = FALSE WHERE converted_price IS NOT NULL AND converted_currency != ''",
+            reverse_sql="UPDATE games_purchase SET needs_price_update = TRUE WHERE converted_price IS NOT NULL AND converted_currency != ''",
+        ),
+    ]