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>

tools/fetch-pokeapi/models.go

@@ -27,6 +27,7 @@ type EvolutionOutput struct {
 	Item          *string `json:"item"`
 	HeldItem      *string `json:"held_item"`
 	Condition     *string `json:"condition"`
+	Region        *string `json:"region"`
 }
 
 type RouteOutput struct {