feat(auth): implement JWT authentication

- Add JwtUtils for token generation and validation using jjwt
- Refactor AuthService.login to return User instead of Boolean
- Add jjwt dependencies and integrate JWT into login flow
- Externalize JWT secret, expiration, and log level as configurable env vars with defaults

build.gradle.kts

@@ -30,6 +30,9 @@ dependencies {
     implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
     implementation("org.springframework.boot:spring-boot-starter-validation")
     implementation("io.github.oshai:kotlin-logging-jvm:7.0.0")
+    implementation("io.jsonwebtoken:jjwt-api:0.13.0")
+    runtimeOnly("io.jsonwebtoken:jjwt-impl:0.13.0")
+    runtimeOnly("io.jsonwebtoken:jjwt-jackson:0.13.0")
     testImplementation("org.springframework.boot:spring-boot-starter-test")
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")
     testRuntimeOnly("org.junit.platform:junit-platform-launcher")