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/api/encounters.ts

@@ -24,6 +24,7 @@ export function deleteEncounter(id: number): Promise<void> {
   return api.del(`/encounters/${id}`)
 }
 
-export function fetchEvolutions(pokemonId: number): Promise<Evolution[]> {
-  return api.get(`/pokemon/${pokemonId}/evolutions`)
+export function fetchEvolutions(pokemonId: number, region?: string): Promise<Evolution[]> {
+  const params = region ? `?region=${encodeURIComponent(region)}` : ''
+  return api.get(`/pokemon/${pokemonId}/evolutions${params}`)
 }