feat: add published_at field support to announcements with scheduled publishing and student visibility filtering

Add published_at field to announcement creation and updates, allowing instructors to schedule announcement publishing. Update student announcement filtering to only show PUBLISHED announcements where published_at <= now. Modify announcement creation to set published_at to provided value or current time for PUBLISHED status. Update announcement updates to handle published_at changes while

Backend/src/controllers/announcementsController.ts

@@ -75,6 +75,7 @@ export class AnnouncementsController {
             content: parsed.content,
             status: parsed.status,
             is_pinned: parsed.is_pinned,
+            published_at: parsed.published_at ? new Date(parsed.published_at) : undefined,
             files,
         });
     }
@@ -107,6 +108,7 @@ export class AnnouncementsController {
             content: body.content,
             status: body.status,
             is_pinned: body.is_pinned,
+            published_at: body.published_at ? new Date(body.published_at) : undefined,
         });
     }