feat(activities): let a group show and limit its members

Empty membership stays open to the whole workspace; picking people restricts who can see that activity group.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/src/activities/dto.ts

@@ -131,6 +131,11 @@ export class CreateActivityBoardDto {
   @MinLength(2)
   @MaxLength(80)
   name!: string;
+
+  @IsOptional()
+  @IsArray()
+  @IsString({ each: true })
+  memberIds?: string[];
 }
 
 export class UpdateActivityBoardDto {
@@ -139,6 +144,11 @@ export class UpdateActivityBoardDto {
   @MinLength(2)
   @MaxLength(80)
   name?: string;
+
+  @IsOptional()
+  @IsArray()
+  @IsString({ each: true })
+  memberIds?: string[];
 }
 
 export class ListActivityStagesQueryDto {