feat: add security response headers (HSTS, X-Frame-Options, nosniff, referrer)

Defense-in-depth headers added via tower-http SetResponseHeaderLayer:
- Strict-Transport-Security: max-age=31536000; includeSubDomains
- X-Frame-Options: DENY
- X-Content-Type-Options: nosniff
- Referrer-Policy: strict-origin-when-cross-origin

Primary enforcement should still be at the Traefik/reverse proxy level.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compliance-agent/Cargo.toml

@@ -25,7 +25,7 @@ uuid = { workspace = true }
 secrecy = { workspace = true }
 regex = { workspace = true }
 axum = "0.8"
-tower-http = { version = "0.6", features = ["cors", "trace"] }
+tower-http = { version = "0.6", features = ["cors", "trace", "set-header"] }
 git2 = "0.20"
 octocrab = "0.44"
 tokio-cron-scheduler = "0.13"

compliance-agent/src/api/server.rs

@@ -1,8 +1,10 @@
 use std::sync::Arc;
 
+use axum::http::HeaderValue;
 use axum::{middleware, Extension};
 use tokio::sync::RwLock;
 use tower_http::cors::CorsLayer;
+use tower_http::set_header::SetResponseHeaderLayer;
 use tower_http::trace::TraceLayer;
 
 use crate::agent::ComplianceAgent;
@@ -14,7 +16,24 @@ pub async fn start_api_server(agent: ComplianceAgent, port: u16) -> Result<(), A
     let mut app = routes::build_router()
         .layer(Extension(Arc::new(agent.clone())))
         .layer(CorsLayer::permissive())
-        .layer(TraceLayer::new_for_http());
+        .layer(TraceLayer::new_for_http())
+        // Security headers (defense-in-depth, primary enforcement via Traefik)
+        .layer(SetResponseHeaderLayer::overriding(
+            axum::http::header::STRICT_TRANSPORT_SECURITY,
+            HeaderValue::from_static("max-age=31536000; includeSubDomains"),
+        ))
+        .layer(SetResponseHeaderLayer::overriding(
+            axum::http::header::X_FRAME_OPTIONS,
+            HeaderValue::from_static("DENY"),
+        ))
+        .layer(SetResponseHeaderLayer::overriding(
+            axum::http::header::X_CONTENT_TYPE_OPTIONS,
+            HeaderValue::from_static("nosniff"),
+        ))
+        .layer(SetResponseHeaderLayer::overriding(
+            axum::http::header::REFERRER_POLICY,
+            HeaderValue::from_static("strict-origin-when-cross-origin"),
+        ));
 
     if let (Some(kc_url), Some(kc_realm)) =
         (&agent.config.keycloak_url, &agent.config.keycloak_realm)