Fix sample.yaml fixture for current schema

Three issues from when the fixture was created before schema evolved:
- Game and Platform lacked created_at (auto_now_add bypassed by loaddata)
- Purchase lacked created_at/updated_at
- Purchase used 'game' FK that no longer exists; field is now the M2M
  'games', serialized as a list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

games/fixtures/sample.yaml

@@ -1,71 +1,90 @@
-- model: games.game
-  pk: 1
-  fields:
-    name: Nioh 2
-    wikidata: Q67482292
-- model: games.game
-  pk: 2
-  fields:
-    name: Elden Ring
-    wikidata: Q64826862
-- model: games.game
-  pk: 3
-  fields:
-    name: Cyberpunk 2077
-    wikidata: Q3182559
-- model: games.purchase
-  pk: 1
-  fields:
-    game: 1
-    platform: 1
-    date_purchased: 2021-02-13
-    date_refunded: null
-- model: games.purchase
-  pk: 2
-  fields:
-    game: 2
-    platform: 1
-    date_purchased: 2022-02-24
-    date_refunded: null
-- model: games.purchase
-  pk: 3
-  fields:
-    game: 3
-    platform: 1
-    date_purchased: 2020-12-07
-    date_refunded: null
 - model: games.platform
   pk: 1
   fields:
     name: Steam
     group: PC
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 3
   fields:
     name: Xbox Gamepass
     group: PC
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 4
   fields:
     name: Epic Games Store
     group: PC
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 5
   fields:
     name: Playstation 5
     group: Playstation
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 6
   fields:
     name: Playstation 4
     group: Playstation
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 7
   fields:
     name: Nintendo Switch
     group: Nintendo
+    created_at: "2020-01-01T00:00:00Z"
 - model: games.platform
   pk: 8
   fields:
     name: Nintendo 3DS
     group: Nintendo
+    created_at: "2020-01-01T00:00:00Z"
+- model: games.game
+  pk: 1
+  fields:
+    name: Nioh 2
+    wikidata: Q67482292
+    created_at: "2021-02-13T00:00:00Z"
+    updated_at: "2021-02-13T00:00:00Z"
+- model: games.game
+  pk: 2
+  fields:
+    name: Elden Ring
+    wikidata: Q64826862
+    created_at: "2022-02-24T00:00:00Z"
+    updated_at: "2022-02-24T00:00:00Z"
+- model: games.game
+  pk: 3
+  fields:
+    name: Cyberpunk 2077
+    wikidata: Q3182559
+    created_at: "2020-12-07T00:00:00Z"
+    updated_at: "2020-12-07T00:00:00Z"
+- model: games.purchase
+  pk: 1
+  fields:
+    games: [1]
+    platform: 1
+    date_purchased: 2021-02-13
+    date_refunded: null
+    created_at: "2021-02-13T00:00:00Z"
+    updated_at: "2021-02-13T00:00:00Z"
+- model: games.purchase
+  pk: 2
+  fields:
+    games: [2]
+    platform: 1
+    date_purchased: 2022-02-24
+    date_refunded: null
+    created_at: "2022-02-24T00:00:00Z"
+    updated_at: "2022-02-24T00:00:00Z"
+- model: games.purchase
+  pk: 3
+  fields:
+    games: [3]
+    platform: 1
+    date_purchased: 2020-12-07
+    date_refunded: null
+    created_at: "2020-12-07T00:00:00Z"
+    updated_at: "2020-12-07T00:00:00Z"