Fix WebSocket price stream by guarding nil responses from ring-defaults

ring-defaults middleware (wrap-content-type, wrap-xss-protection) was
converting the nil response from the raw WebSocket upgrade handler into
a partial response map (headers only, no status), causing the Undertow
adapter to attempt ending the already-upgraded exchange.

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

src/clj/pmagnus/btcdata/web/middleware/core.clj

@@ -16,6 +16,14 @@
         (update resp :headers assoc
                 "Access-Control-Allow-Origin" origin)))))
 
+(defn- wrap-nil-guard
+  "Discard responses with no :status — these are artifacts of ring-defaults
+   middleware wrapping a nil response (e.g., from a raw WebSocket upgrade)."
+  [handler]
+  (fn [request]
+    (let [resp (handler request)]
+      (when (:status resp) resp))))
+
 (defn wrap-base [{:keys [site-defaults-config cors-origin]}]
   (let [origin (or cors-origin
                    (System/getenv "CORS_ORIGIN")
@@ -24,4 +32,5 @@
       (-> handler
           (defaults/wrap-defaults
            (or site-defaults-config defaults/api-defaults))
+          (wrap-nil-guard)
           (wrap-cors origin)))))