Anchor DLC purchases to a base game instead of a parent purchase

Add-on purchases (DLC, Season Pass, Battle Pass) previously linked to a
parent *purchase* via the `related_purchase` self-FK. When the base game
was bought inside a multi-game purchase (e.g. a bundle), there was no
per-game purchase to point at — only the whole bundle.

Replace it with a `related_game` FK (Game -> Game): an add-on belongs to
a *game*, which is unambiguous regardless of how the base game was bought.

- models: drop `related_purchase`; add `related_game`
  (SET_NULL, related_name="addon_purchases"); require it for non-GAME
  types in `save()`.
- forms: replace the parent-purchase picker with a flat `related_game`
  game search (reusing SearchSelectWidget/_game_options); drop the now
  unused related_purchase_queryset/RelatedPurchaseChoiceField.
- views/urls: remove the obsolete related_purchase_by_game endpoint.
- add_purchase.js: drop the parent-dropdown refetch; keep platform
  auto-fill; retarget the type toggle to #id_related_game.
- migration 0020: add -> backfill (related_game = parent's first game by
  sort_name) -> remove related_purchase.
- tests: model validation unit tests + an e2e test for the flat picker.

related_game is deliberately game->game so it can later be synced from
IGDB's parent_game without schema changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

games/migrations/0020_remove_purchase_related_purchase_and_more.py

@@ -0,0 +1,46 @@
+# Generated by Django 6.0.6 on 2026-06-18 21:03
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+def backfill_related_game(apps, schema_editor):
+    """Move each add-on purchase's parent link from the parent *purchase* to a
+    parent *game*. For a parent bought as a multi-game bundle there is no single
+    game, so use the bundle's first game (by sort_name) as the best guess."""
+    Purchase = apps.get_model("games", "Purchase")
+    for purchase in Purchase.objects.filter(related_purchase__isnull=False):
+        parent_game = purchase.related_purchase.games.order_by("sort_name").first()
+        if parent_game is not None:
+            purchase.related_game = parent_game
+            purchase.save(update_fields=["related_game"])
+
+
+def noop_reverse(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("games", "0019_alter_filterpreset_mode"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="purchase",
+            name="related_game",
+            field=models.ForeignKey(
+                blank=True,
+                default=None,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="addon_purchases",
+                to="games.game",
+            ),
+        ),
+        migrations.RunPython(backfill_related_game, noop_reverse),
+        migrations.RemoveField(
+            model_name="purchase",
+            name="related_purchase",
+        ),
+    ]