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/loader.py

@@ -184,6 +184,7 @@ async def upsert_evolutions(
             item=evo.get("item"),
             held_item=evo.get("held_item"),
             condition=evo.get("condition"),
+            region=evo.get("region"),
         )
         session.add(evolution)
         count += 1