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>

tests/test_purchase_related_game.py

@@ -0,0 +1,50 @@
+from datetime import date
+
+from django.core.exceptions import ValidationError
+from django.test import TestCase
+
+from games.models import Game, Platform, Purchase
+
+
+class PurchaseRelatedGameTest(TestCase):
+    def setUp(self):
+        self.platform = Platform.objects.create(name="PC", icon="pc", group="PC")
+        self.base_game = Game.objects.create(name="Base Game", platform=self.platform)
+        self.dlc_game = Game.objects.create(name="The DLC", platform=self.platform)
+
+    def test_non_game_purchase_requires_related_game(self):
+        purchase = Purchase(
+            price=10.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+            type=Purchase.SEASONPASS,
+            name="Season Pass",
+        )
+        with self.assertRaises(ValidationError):
+            purchase.save()
+
+    def test_non_game_purchase_saves_with_related_game(self):
+        purchase = Purchase(
+            price=10.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+            type=Purchase.SEASONPASS,
+            name="Season Pass",
+            related_game=self.base_game,
+        )
+        purchase.save()
+        purchase.games.add(self.dlc_game)
+
+        self.assertEqual(purchase.related_game, self.base_game)
+        # Reverse accessor: the base game lists its add-on purchases.
+        self.assertIn(purchase, self.base_game.addon_purchases.all())
+
+    def test_plain_game_purchase_needs_no_related_game(self):
+        purchase = Purchase(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+            type=Purchase.GAME,
+        )
+        purchase.save()  # must not raise
+        self.assertIsNone(purchase.related_game)