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/forms.py

@@ -1,6 +1,5 @@
 from django import forms
 from django.db import transaction
-from django.db.models import OuterRef, Subquery
 
 from common.components import (
     DEFAULT_PREFETCH,
@@ -228,31 +227,6 @@ class SessionForm(PrimitiveWidgetsMixin, forms.ModelForm):
         return session
 
 
-def related_purchase_queryset():
-    """GAME purchases annotated with their first game's name.
-
-    Rendering the ``related_purchase`` ``<select>`` calls ``str()`` on every
-    option, and ``Purchase.__str__`` falls back to ``first_game`` — one extra
-    query per option (700+ on a large library). Annotating the first game's
-    name via a subquery lets the choice field build labels without those
-    per-row queries.
-    """
-    first_game_name = Subquery(
-        Game.objects.filter(purchases=OuterRef("pk")).order_by("id").values("name")[:1]
-    )
-    return Purchase.objects.filter(type=Purchase.GAME).annotate(
-        _first_game_name=first_game_name
-    )
-
-
-class RelatedPurchaseChoiceField(forms.ModelChoiceField):
-    def label_from_instance(self, obj) -> str:
-        # Mirrors Purchase.standardized_name but reads the annotated first-game
-        # name instead of querying first_game per option.
-        name = obj.name or getattr(obj, "_first_game_name", None)
-        return name or obj.standardized_name
-
-
 class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
@@ -272,9 +246,12 @@ class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
             search_url="/api/platforms/search", options_resolver=_platform_options
         ),
     )
-    related_purchase = RelatedPurchaseChoiceField(
-        queryset=related_purchase_queryset(),
+    related_game = forms.ModelChoiceField(
+        queryset=Game.objects.order_by("sort_name"),
         required=False,
+        widget=SearchSelectWidget(
+            search_url="/api/games/search", options_resolver=_game_options
+        ),
     )
 
     price_currency = forms.CharField(
@@ -305,14 +282,14 @@ class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
             "price_currency",
             "ownership_type",
             "type",
-            "related_purchase",
+            "related_game",
             "name",
         ]
 
     def clean(self):
         cleaned_data = super().clean()
         purchase_type = cleaned_data.get("type")
-        related_purchase = cleaned_data.get("related_purchase")
+        related_game = cleaned_data.get("related_game")
         name = cleaned_data.get("name")
 
         # Set the type on the instance to use get_type_display()
@@ -321,10 +298,10 @@ class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
 
         if purchase_type != Purchase.GAME:
             type_display = self.instance.get_type_display()
-            if not related_purchase:
+            if not related_game:
                 self.add_error(
-                    "related_purchase",
-                    f"{type_display} must have a related purchase.",
+                    "related_game",
+                    f"{type_display} must have a related game.",
                 )
             if not name:
                 self.add_error("name", f"{type_display} must have a name.")