fix: update JwtService to handle default expiration and add tests for token generation

backend/src/test/kotlin/com/condado/newsletter/service/AuthServiceTest.kt

@@ -40,7 +40,7 @@ class AuthServiceTest {
     fun should_returnValidClaims_when_jwtTokenParsed() {
         val realJwtService = JwtService(
             secret = "test-secret-key-for-testing-only-must-be-at-least-32-characters",
-            expirationMs = 86400000L
+            expirationMsRaw = "86400000"
         )
         val token = realJwtService.generateToken()
 
@@ -51,7 +51,7 @@ class AuthServiceTest {
     fun should_returnFalse_when_expiredTokenValidated() {
         val realJwtService = JwtService(
             secret = "test-secret-key-for-testing-only-must-be-at-least-32-characters",
-            expirationMs = 1L
+            expirationMsRaw = "1"
         )
         val token = realJwtService.generateToken()