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/static/js/add_purchase.js

@@ -1,40 +1,23 @@
 import { getEl, disableElementsWhenTrue, onSwap } from "./utils.js";
 
-const RELATED_PURCHASE_URL = "/tracker/purchase/related-purchase-by-game";
-
 // The games field is now a SearchSelect widget (a <div>, not a <select>), so we
 // react to its custom "search-select:change" event instead of syncing a select.
 document.addEventListener("search-select:change", (event) => {
   if (event.detail.name !== "games") return;
 
-  // (a) Auto-fill platform from the clicked option's data-platform.
+  // Auto-fill platform from the clicked option's data-platform.
   const last = event.detail.last;
   const platformId = last && last.data ? last.data.platform : "";
   if (platformId) {
     const platformEl = getEl("#id_platform");
     if (platformEl) platformEl.value = platformId;
   }
-
-  // (b) Refresh #id_related_purchase for the currently selected games.
-  const query = event.detail.values
-    .map((value) => "games=" + encodeURIComponent(value))
-    .join("&");
-  fetch(RELATED_PURCHASE_URL + "?" + query, { credentials: "same-origin" })
-    .then((response) => {
-      if (response.status === 204) return null;
-      return response.text();
-    })
-    .then((html) => {
-      if (html === null) return;
-      const target = getEl("#id_related_purchase");
-      if (target) target.outerHTML = html;
-    });
 });
 
 function setupElementHandlers() {
   disableElementsWhenTrue("#id_type", "game", [
     "#id_name",
-    "#id_related_purchase",
+    "#id_related_game",
   ]);
 }