fix: allow editing expenses in pre-migration events (empty month_id)

internal/handlers/helpers.go

@@ -15,10 +15,11 @@ func normalizeImagePath(path string) string {
 }
 
 // verifyMonthOwnership checks that a month exists and belongs to the given user.
-// Returns true if the month is owned by the user, false otherwise.
+// If monthID is empty (pre-migration events), returns true since event ownership
+// was already verified by the caller.
 func verifyMonthOwnership(db *sql.DB, monthID, userID string) bool {
 	if monthID == "" {
-		return false
+		return true
 	}
 	month, err := database.GetMonthByID(db, monthID)
 	if err != nil || month == nil {