fix(sse): send plain string body for subsequent SSE events

http-kit's send! after the initial response expects a plain string,
not a full response map with status/headers. Sending a response map
was silently dropped, so SSE clients never received change
notifications after the initial "connected" message.

Verified working: toggling a todo in one tab updates the other tab
within ~30ms via SSE.

src/pocketbook/server.clj

@@ -66,15 +66,12 @@
       (swap! sse-clients update group disj ch))))
 
 (defn- sse-notify!
-  "Send an SSE event to all listeners of the given groups."
+  "Send an SSE event to all listeners of the given groups.
+   After the initial response, http-kit send! takes a plain string body."
   [groups]
   (doseq [group groups
           ch (get @sse-clients group)]
-    (http/send! ch {:status 200
-                    :headers {"Content-Type"  "text/event-stream"
-                              "Cache-Control" "no-cache"}
-                    :body    (str "data: " group "\n\n")}
-                false)))  ;; false = don't close, keep streaming
+    (http/send! ch (str "data: " group "\n\n") false)))
 
 ;; ---------------------------------------------------------------------------
 ;; Handlers