test(auth): integration tests for full session lifecycle and idle-timeout

Also switches pom.xml to spring-boot-starter-session-jdbc (Spring Boot 4.x
split the session auto-config into a separate starter; spring-session-jdbc
alone does not register JdbcSessionAutoConfiguration).
Adds SpringSessionConfig#cookieSerializer bean to configure fa_session name
and SameSite=Strict (spring.session.cookie.* properties are no longer
supported by the Boot 4.x auto-configuration layer).
Cleans up application.yaml / application-dev.yaml: removes store-type: jdbc
and the unsupported cookie.* keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/config/SpringSessionConfig.java

@@ -0,0 +1,22 @@
+package org.raddatz.familienarchiv.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.session.web.http.CookieSerializer;
+import org.springframework.session.web.http.DefaultCookieSerializer;
+
+@Configuration
+public class SpringSessionConfig {
+
+    @Bean
+    public CookieSerializer cookieSerializer() {
+        DefaultCookieSerializer serializer = new DefaultCookieSerializer();
+        serializer.setCookieName("fa_session");
+        serializer.setSameSite("Strict");
+        // cookieHttpOnly: true is the DefaultCookieSerializer default
+        // useSecureCookie not set: auto-detects from request.isSecure().
+        // With forward-headers-strategy: native, Caddy's X-Forwarded-Proto: https
+        // causes isSecure() → true in production; direct HTTP in dev/tests → false.
+        return serializer;
+    }
+}