fix: add spring-boot-starter-validation for Jakarta Bean Validation provider

Core service returned 500 on /api/auth/register and /api/auth/login
because no Jakarta Bean Validation provider was on the classpath.
This blocked all authenticated API endpoints.

pom.xml

@@ -31,6 +31,11 @@
 		<java.version>21</java.version>
 	</properties>
 	<dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-validation</artifactId>
+        </dependency>
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter</artifactId>