Return 404 when updating max_devices for non-existent user

- Check if returned user record is null (user not found)
- Return 404 Not Found instead of 200 OK
- Provides accurate REST API semantics
- Fixes TestUpdateUserMaxDevicesNonExistentUser

Related: Database query change commit

internal/handlers/auth.go

@@ -776,11 +776,14 @@ func (h *AuthHandler) UpdateUserMaxDevices(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
 	}
 
-	err = h.db.UpdateUserMaxDevices(c.Request().Context(), database.UpdateUserMaxDevicesParams{
+	user, err := h.db.UpdateUserMaxDevices(c.Request().Context(), database.UpdateUserMaxDevicesParams{
 		ID:         pgtype.UUID{Bytes: userUUID, Valid: true},
 		MaxDevices: pgtype.Int4{Int32: req.MaxDevices, Valid: true},
 	})
 	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "user not found"})
+		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}