fix(backend): Handle missing users in feed API gracefully

backend/routers/feed.py

@@ -41,29 +41,53 @@ def get_global_feed(
     
     for r in reviews:
         user = session.get(User, r.user_id)
+        user_data = {"id": 0, "username": "Deleted User", "avatar": None}
+        if user:
+            user_data = {
+                "id": user.id, 
+                "username": user.email.split("@")[0] if user.email else "User", 
+                "avatar": user.avatar
+            }
+        
         feed_items.append(FeedItem(
             type="review",
-            timestamp=r.created_at or datetime.utcnow(), # Handle missing created_at if any
+            timestamp=r.created_at or datetime.utcnow(),
             data=r,
-            user={"id": user.id, "username": user.email.split("@")[0], "avatar": user.avatar} if user else {}
+            user=user_data
         ))
         
     for a in attendance:
         user = session.get(User, a.user_id)
+        user_data = {"id": 0, "username": "Deleted User", "avatar": None}
+        if user:
+            user_data = {
+                "id": user.id, 
+                "username": user.email.split("@")[0] if user.email else "User", 
+                "avatar": user.avatar
+            }
+
         feed_items.append(FeedItem(
             type="attendance",
             timestamp=a.created_at,
             data=a,
-            user={"id": user.id, "username": user.email.split("@")[0], "avatar": user.avatar} if user else {}
+            user=user_data
         ))
         
     for p in posts:
         user = session.get(User, p.user_id)
+        user_data = {"id": 0, "username": "Deleted User", "avatar": None}
+        if user:
+            user_data = {
+                "id": user.id, 
+                "username": user.email.split("@")[0] if user.email else "User", 
+                "avatar": user.avatar
+            }
+
         feed_items.append(FeedItem(
             type="post",
             timestamp=p.created_at,
             data=p,
-            user={"id": user.id, "username": user.email.split("@")[0], "avatar": user.avatar} if user else {}
+            user=user_data
         ))
         
     # Sort by timestamp desc