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/hooks/useEncounters.ts

@@ -43,10 +43,10 @@ export function useDeleteEncounter(runId: number) {
   })
 }
 
-export function useEvolutions(pokemonId: number | null) {
+export function useEvolutions(pokemonId: number | null, region?: string) {
   return useQuery({
-    queryKey: ['evolutions', pokemonId],
-    queryFn: () => fetchEvolutions(pokemonId!),
+    queryKey: ['evolutions', pokemonId, region],
+    queryFn: () => fetchEvolutions(pokemonId!, region),
     enabled: pokemonId !== null,
   })
 }