feat(activities): let creators lock edit and delete to themselves

Add editPolicy (default creator-only), enforce it in the API, surface clear permission messages, and drop the redundant edit menu now that detail is editable.

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

apps/api/src/activities/dto.ts

@@ -388,6 +388,10 @@ export class CreateActivityDto {
   @IsOptional()
   @IsIn(['any', 'all'])
   completionMode?: 'any' | 'all';
+
+  @IsOptional()
+  @IsIn(['creator', 'assignees'])
+  editPolicy?: 'creator' | 'assignees';
 }
 
 export class UpdateActivityDto {
@@ -492,4 +496,8 @@ export class UpdateActivityDto {
   @IsOptional()
   @IsIn(['any', 'all'])
   completionMode?: 'any' | 'all';
+
+  @IsOptional()
+  @IsIn(['creator', 'assignees'])
+  editPolicy?: 'creator' | 'assignees';
 }