feat(security): trust X-Forwarded-Proto behind reverse proxy

Adds server.forward-headers-strategy: native so that Jetty honours
X-Forwarded-{Proto,For,Host} from Caddy. Without this, getScheme(),
redirect URLs, and Spring Session "Secure" cookies reflect the
internal http hop instead of the original https client request.

Refs #497.

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

backend/src/main/resources/application.yaml

@@ -38,6 +38,12 @@ spring:
           starttls:
             enable: true
 
+server:
+  # Behind Caddy/reverse proxy: trust X-Forwarded-{Proto,For,Host} so that
+  # request.getScheme(), redirect URLs, and Spring Session "Secure" cookies
+  # reflect the original https client request, not the http hop from Caddy.
+  forward-headers-strategy: native
+
 management:
   health:
     mail:

backend/src/test/java/org/raddatz/familienarchiv/config/ForwardHeadersConfigurationTest.java

@@ -0,0 +1,37 @@
+package org.raddatz.familienarchiv.config;
+
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.PostgresContainerConfig;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import software.amazon.awssdk.services.s3.S3Client;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
+@ActiveProfiles("test")
+@Import(PostgresContainerConfig.class)
+class ForwardHeadersConfigurationTest {
+
+    @MockitoBean
+    S3Client s3Client;
+
+    @Autowired
+    @Value("${server.forward-headers-strategy:}")
+    String forwardHeadersStrategy;
+
+    @Test
+    void forward_headers_strategy_is_native_for_reverse_proxy_deployment() {
+        // Caddy terminates TLS and forwards X-Forwarded-Proto: https.
+        // Spring must trust those headers so that AppUser-facing redirect URLs,
+        // Spring Session cookies (Secure flag), and HttpServletRequest.getScheme()
+        // reflect the original client-facing scheme rather than the internal http hop.
+        assertThat(forwardHeadersStrategy)
+            .as("server.forward-headers-strategy must be 'native' so Jetty honours X-Forwarded-Proto behind Caddy")
+            .isEqualTo("native");
+    }
+}