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

backend/src/main/kotlin/com/condado/newsletter/service/JwtService.kt

@@ -14,8 +14,10 @@ import java.util.Date
 @Service
 class JwtService(
     @Value("\${app.jwt.secret}") val secret: String,
-    @Value("\${app.jwt.expiration-ms}") val expirationMs: Long
+    @Value("\${app.jwt.expiration-ms:86400000}") expirationMsRaw: String
 ) {
+    private val expirationMs: Long = expirationMsRaw.toLongOrNull() ?: 86400000L
+
     private val signingKey by lazy {
         Keys.hmacShaKeyFor(secret.toByteArray(Charsets.UTF_8))
     }