feat(mcp): origin allowlist middleware

internal/mcp/origin.go

@@ -0,0 +1,27 @@
+package mcp
+
+import "net/http"
+
+// OriginAllowlist returns middleware that rejects requests whose Origin header
+// is not in the allowlist. Empty Origin (e.g. server-side curl) is allowed
+// because Origin is browser-only by design.
+func OriginAllowlist(allowed []string) func(http.Handler) http.Handler {
+	set := make(map[string]struct{}, len(allowed))
+	for _, a := range allowed {
+		set[a] = struct{}{}
+	}
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			origin := r.Header.Get("Origin")
+			if origin == "" {
+				next.ServeHTTP(w, r)
+				return
+			}
+			if _, ok := set[origin]; !ok {
+				http.Error(w, "origin not allowed", http.StatusForbidden)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}