Add nullable region field to evolutions for regional form filtering

Regional evolutions (e.g., Pikachu → Alolan Raichu) only occur in specific
regions. This adds a nullable region column so the app can filter evolutions
by the game's region. When a regional evolution exists for a given trigger/item,
the non-regional counterpart is automatically hidden.

Full-stack: migration, model, schemas, API with region query param, seeder,
Go fetch tool, frontend types/API/hook/components, and admin form.

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

backend/src/app/seeds/data/evolution_overrides.json

@@ -1,5 +1,42 @@
 {
   "remove": [],
-  "add": [],
+  "add": [
+    {
+      "from_dex": 25,
+      "to_dex": 10100,
+      "trigger": "use-item",
+      "item": "thunder-stone",
+      "region": "alola"
+    },
+    {
+      "from_dex": 102,
+      "to_dex": 10101,
+      "trigger": "use-item",
+      "item": "leaf-stone",
+      "region": "alola"
+    },
+    {
+      "from_dex": 77,
+      "to_dex": 10162,
+      "trigger": "level-up",
+      "min_level": 40,
+      "region": "galar"
+    },
+    {
+      "from_dex": 263,
+      "to_dex": 10176,
+      "trigger": "level-up",
+      "min_level": 20,
+      "region": "galar"
+    },
+    {
+      "from_dex": 10176,
+      "to_dex": 10177,
+      "trigger": "level-up",
+      "min_level": 35,
+      "condition": "nighttime",
+      "region": "galar"
+    }
+  ],
   "modify": []
 }