feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/libs/services/upload.go

@@ -384,15 +384,27 @@ func (s *UploadService) UserBoxes(userID string, collectionNames map[string]stri
 }
 
 func (s *UploadService) UserStorageUsed(userID string) (int64, error) {
+	return s.userStorageUsed(userID, false)
+}
+
+func (s *UploadService) UserActiveStorageUsed(userID string) (int64, error) {
+	return s.userStorageUsed(userID, true)
+}
+
+func (s *UploadService) userStorageUsed(userID string, activeOnly bool) (int64, error) {
 	boxes, err := s.ListBoxes(0)
 	if err != nil {
 		return 0, err
 	}
 	var total int64
+	now := time.Now().UTC()
 	for _, box := range boxes {
 		if box.OwnerID != userID {
 			continue
 		}
+		if activeOnly && !box.ExpiresAt.After(now) {
+			continue
+		}
 		for _, file := range box.Files {
 			total += file.Size
 		}