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/pages/RunDashboard.tsx

@@ -242,6 +242,7 @@ export function RunDashboard() {
           }}
           onClose={() => setSelectedEncounter(null)}
           isPending={updateEncounter.isPending}
+          region={run?.game.region}
         />
       )}
 

frontend/src/pages/RunEncounters.tsx

@@ -748,6 +748,7 @@ export function RunEncounters() {
           }}
           onClose={() => setSelectedTeamEncounter(null)}
           isPending={updateEncounter.isPending}
+          region={run?.game.region}
         />
       )}