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/evolutions.go

@@ -165,6 +165,7 @@ type EvolutionOverrides struct {
 		Item      *string `json:"item"`
 		HeldItem  *string `json:"held_item"`
 		Condition *string `json:"condition"`
+		Region    *string `json:"region"`
 	} `json:"add"`
 	Modify []struct {
 		FromDex int               `json:"from_dex"`
@@ -213,6 +214,7 @@ func applyEvolutionOverrides(evolutions []EvolutionOutput, overridesPath string)
 			Item:          addition.Item,
 			HeldItem:      addition.HeldItem,
 			Condition:     addition.Condition,
+			Region:        addition.Region,
 		})
 	}
 
@@ -252,6 +254,12 @@ func applyEvolutionOverrides(evolutions []EvolutionOutput, overridesPath string)
 						} else if value == nil {
 							e.Condition = nil
 						}
+					case "region":
+						if s, ok := value.(string); ok {
+							e.Region = &s
+						} else if value == nil {
+							e.Region = nil
+						}
 					}
 				}
 			}