Serve sled binary artifacts from ~/.huskies/artifacts/

GET /api/artifacts/:filename with filename validation (no path
components, no dotfiles). Sleds only ever download binaries from their
own gateway; this endpoint is where the gateway serves them from,
replacing the current_exe()-based /api/huskies-binary which serves the
gateway's own (macOS) binary — wrong platform for Linux sleds.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fHdm92yjvguPi2LiXfLB9

server/src/http/mod.rs

@@ -124,7 +124,8 @@ pub fn build_routes(
 
     route = route
         .at("/api/upgrade", post(upgrade_trigger_handler))
-        .at("/api/huskies-binary", get(serve_binary_handler));
+        .at("/api/huskies-binary", get(serve_binary_handler))
+        .at("/api/artifacts/:filename", get(serve_artifact_handler));
 
     if let Some(wa_ctx) = whatsapp_ctx {
         route = route.at(
@@ -297,6 +298,56 @@ pub async fn serve_binary_handler() -> poem::Response {
     }
 }
 
+/// Directory where the gateway stores distributable sled binaries.
+///
+/// Host-global (`~/.huskies/artifacts/`), not per-project: one artifact serves
+/// every sled behind this gateway.
+pub fn artifacts_dir() -> PathBuf {
+    let home = std::env::var("HOME").unwrap_or_else(|_| "/home/huskies".to_string());
+    PathBuf::from(home).join(".huskies").join("artifacts")
+}
+
+/// Validate an artifact filename: plain name, no path components.
+fn is_valid_artifact_name(name: &str) -> bool {
+    !name.is_empty()
+        && !name.starts_with('.')
+        && name
+            .chars()
+            .all(|c| c.is_ascii_alphanumeric() || matches!(c, '-' | '_' | '.'))
+}
+
+/// `GET /api/artifacts/:filename` — serve a published sled binary.
+///
+/// The gateway is the only place sleds download binaries from; where the
+/// artifact came from (local build, release channel) is the gateway's concern.
+/// Returns 400 for names with path components, 404 when the artifact does not
+/// exist.
+#[poem::handler]
+pub async fn serve_artifact_handler(
+    poem::web::Path(filename): poem::web::Path<String>,
+) -> poem::Response {
+    if !is_valid_artifact_name(&filename) {
+        return poem::Response::builder()
+            .status(StatusCode::BAD_REQUEST)
+            .body("Invalid artifact name");
+    }
+
+    let path = artifacts_dir().join(&filename);
+    match tokio::fs::read(&path).await {
+        Ok(bytes) => poem::Response::builder()
+            .status(StatusCode::OK)
+            .header("Content-Type", "application/octet-stream")
+            .header(
+                "Content-Disposition",
+                format!("attachment; filename=\"{filename}\""),
+            )
+            .body(bytes),
+        Err(_) => poem::Response::builder()
+            .status(StatusCode::NOT_FOUND)
+            .body(format!("No artifact named {filename}")),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -353,6 +404,35 @@ mod tests {
         let _endpoint = build_routes(ctx, None, None, 3001, None);
     }
 
+    #[test]
+    fn artifact_name_validation() {
+        assert!(is_valid_artifact_name("huskies-linux-arm64"));
+        assert!(is_valid_artifact_name("huskies-linux-amd64.v2"));
+        assert!(!is_valid_artifact_name(""));
+        assert!(!is_valid_artifact_name(".."));
+        assert!(!is_valid_artifact_name(".hidden"));
+        assert!(!is_valid_artifact_name("a/b"));
+        assert!(!is_valid_artifact_name("../../etc/passwd"));
+        assert!(!is_valid_artifact_name("name with spaces"));
+    }
+
+    #[tokio::test]
+    async fn artifact_endpoint_rejects_traversal_and_misses() {
+        let tmp = tempfile::tempdir().unwrap();
+        let ctx = context::AppContext::new_test(tmp.path().to_path_buf());
+        let app = build_routes(ctx, None, None, 3001, None);
+        let cli = poem::test::TestClient::new(app);
+
+        let resp = cli.get("/api/artifacts/..").send().await;
+        assert_eq!(resp.0.status(), StatusCode::BAD_REQUEST);
+
+        let resp = cli
+            .get("/api/artifacts/definitely-not-a-real-artifact-xyz9")
+            .send()
+            .await;
+        assert_eq!(resp.0.status(), StatusCode::NOT_FOUND);
+    }
+
     #[tokio::test]
     async fn version_endpoint_reports_version_and_git_hash() {
         let tmp = tempfile::tempdir().unwrap();