Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/internal/calendar/attachments.go

@@ -137,6 +137,26 @@ func (s Store) FindAttachment(ctx context.Context, id string, viewerUserID strin
 	return scanAttachment(row)
 }
 
+func (s Store) DeleteAttachment(ctx context.Context, id string) (Attachment, error) {
+	row := s.db.QueryRow(ctx, `
+		DELETE FROM calendar_item_attachments
+		WHERE id = $1::uuid
+		RETURNING
+			id::text,
+			calendar_item_id::text,
+			original_filename,
+			stored_filename,
+			mime_type,
+			size_bytes,
+			storage_driver,
+			storage_path,
+			created_by::text,
+			created_at
+	`, id)
+
+	return scanAttachment(row)
+}
+
 type attachmentScanner interface {
 	Scan(dest ...any) error
 }