Add X-TS-Version and X-TS-Env response headers for staging identification

Expose FASTLY_SERVICE_VERSION as X-TS-Version on every response, and set
X-TS-Env: staging when FASTLY_IS_STAGING=1. This makes it trivial to tell
staging from production via curl or browser DevTools.

Adds constants for both the Fastly env vars and the header names.

Closes #325

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

Author: aram356

crates/common/src/constants.rs

@@ -19,6 +19,14 @@ pub const HEADER_X_REQUEST_ID: HeaderName = HeaderName::from_static("x-request-i
 pub const HEADER_X_COMPRESS_HINT: HeaderName = HeaderName::from_static("x-compress-hint");
 pub const HEADER_X_DEBUG_FASTLY_POP: HeaderName = HeaderName::from_static("x-debug-fastly-pop");
 
+// Staging / version identification headers
+pub const HEADER_X_TS_VERSION: HeaderName = HeaderName::from_static("x-ts-version");
+pub const HEADER_X_TS_ENV: HeaderName = HeaderName::from_static("x-ts-env");
+
+// Fastly environment variables
+pub const ENV_FASTLY_SERVICE_VERSION: &str = "FASTLY_SERVICE_VERSION";
+pub const ENV_FASTLY_IS_STAGING: &str = "FASTLY_IS_STAGING";
+
 // Common standard header names used across modules
 pub const HEADER_USER_AGENT: HeaderName = HeaderName::from_static("user-agent");
 pub const HEADER_ACCEPT: HeaderName = HeaderName::from_static("accept");

crates/fastly/src/main.rs

@@ -6,6 +6,9 @@ use log_fastly::Logger;
 use trusted_server_common::auction::endpoints::handle_auction;
 use trusted_server_common::auction::{build_orchestrator, AuctionOrchestrator};
 use trusted_server_common::auth::enforce_basic_auth;
+use trusted_server_common::constants::{
+    ENV_FASTLY_IS_STAGING, ENV_FASTLY_SERVICE_VERSION, HEADER_X_TS_ENV, HEADER_X_TS_VERSION,
+};
 use trusted_server_common::error::TrustedServerError;
 use trusted_server_common::integrations::IntegrationRegistry;
 use trusted_server_common::proxy::{
@@ -63,7 +66,7 @@ async fn route_request(
 ) -> Result<Response, Error> {
     log::info!(
         "FASTLY_SERVICE_VERSION: {}",
-        ::std::env::var("FASTLY_SERVICE_VERSION").unwrap_or_else(|_| String::new())
+        ::std::env::var(ENV_FASTLY_SERVICE_VERSION).unwrap_or_else(|_| String::new())
     );
 
     if let Some(response) = enforce_basic_auth(settings, &req) {
@@ -132,6 +135,13 @@ async fn route_request(
     // Convert any errors to HTTP error responses
     let mut response = result.unwrap_or_else(|e| to_error_response(&e));
 
+    if let Ok(v) = ::std::env::var(ENV_FASTLY_SERVICE_VERSION) {
+        response.set_header(HEADER_X_TS_VERSION, v);
+    }
+    if ::std::env::var(ENV_FASTLY_IS_STAGING).as_deref() == Ok("1") {
+        response.set_header(HEADER_X_TS_ENV, "staging");
+    }
+
     for (key, value) in &settings.response_headers {
         response.set_header(key, value);
     }