refactored to make use of the ServiceType enum throughout backend and frontend for typesafety

backend/src/main/java/com/vaessl/app/search/SearchRequest.java

@@ -1,7 +1,9 @@
 package com.vaessl.app.search;
 
+import com.vaessl.app.connection.ServiceType;
 import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotNull;
 
 public record SearchRequest(@NotBlank String appUrl, @NotBlank String username, String query,
-        @NotBlank String serviceType) {
+        @NotNull ServiceType serviceType) {
 }