fix: use schema-based deleteKey and reactive key list refresh

- Add deleteKeySchema for proper FormData validation
- Return { deleted: true } from deleteKey for change tracking
- Use $state for keysPromise to refresh list after create/delete

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/lib/schema/api-keys.ts

@@ -3,3 +3,7 @@ import { object, string, pipe, minLength } from 'valibot';
 export const createKeySchema = object({
 	name: pipe(string(), minLength(1))
 });
+
+export const deleteKeySchema = object({
+	keyId: pipe(string(), minLength(1))
+});