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>

frontend/src/components/admin/EvolutionFormModal.tsx

@@ -29,6 +29,7 @@ export function EvolutionFormModal({
   const [item, setItem] = useState(evolution?.item ?? '')
   const [heldItem, setHeldItem] = useState(evolution?.heldItem ?? '')
   const [condition, setCondition] = useState(evolution?.condition ?? '')
+  const [region, setRegion] = useState(evolution?.region ?? '')
 
   const handleSubmit = (e: FormEvent) => {
     e.preventDefault()
@@ -41,6 +42,7 @@ export function EvolutionFormModal({
       item: item || null,
       heldItem: heldItem || null,
       condition: condition || null,
+      region: region || null,
     })
   }
 
@@ -119,6 +121,16 @@ export function EvolutionFormModal({
           className="w-full px-3 py-2 border rounded-md dark:bg-gray-700 dark:border-gray-600"
         />
       </div>
+      <div>
+        <label className="block text-sm font-medium mb-1">Region</label>
+        <input
+          type="text"
+          value={region}
+          onChange={(e) => setRegion(e.target.value)}
+          placeholder="e.g. alola (blank = all regions)"
+          className="w-full px-3 py-2 border rounded-md dark:bg-gray-700 dark:border-gray-600"
+        />
+      </div>
     </FormModal>
   )
 }