feat: add SSE live sync between clients

When any client pushes changes, the server broadcasts an SSE event to
all connected clients watching that group. Clients immediately pull
the latest data — no polling delay, no page reload.

Server (~30 lines):
- GET /events?group=G — SSE endpoint, holds connection open
- On successful POST /sync, notify all SSE listeners for affected groups
- Auth-aware: respects token and group permissions
- Auto-cleanup on disconnect via http-kit on-close

Client (~25 lines):
- EventSource connects to /events?group=G on sync loop start
- On SSE message, triggers do-pull! to fetch latest data
- Auto-reconnects (browser EventSource built-in behavior)
- Cleanup on destroy!

Periodic polling remains as fallback (30s default). SSE provides
sub-second sync for the common case.

src/pocketbook/core.cljs

@@ -275,26 +275,31 @@
 (defn- start-sync-loop!
   "Start the background sync loop. Returns a stop function."
   [sa]
-  (let [stop-ch (.-stop_ch sa)
-        interval (.-sync_interval sa)]
-    ;; Periodic sync
+  (let [stop-ch  (.-stop_ch sa)
+        interval (.-sync_interval sa)
+        cleanups (atom [])]
+    ;; Periodic sync (fallback)
     (go-loop []
       (let [[_ ch] (alts! [stop-ch (timeout interval)])]
         (when-not (= ch stop-ch)
           (<! (do-sync! sa))
           (recur))))
     ;; Online/offline handler
-    (let [cleanup (sync/on-connectivity-change
-                    (fn [] ; online
-                      (go (<! (do-sync! sa))))
-                    (fn [] ; offline
-                      nil))]
-      (reset! (.-cleanup_fn sa) cleanup))
+    (swap! cleanups conj
+      (sync/on-connectivity-change
+        (fn [] (go (<! (do-sync! sa))))
+        (fn [] nil)))
+    ;; SSE — live pull on server push
+    (when-let [opts (.-server_opts sa)]
+      (swap! cleanups conj
+        (sync/listen-events opts (.-group sa)
+          (fn [_group]
+            (go (<! (do-pull! sa)))))))
+    (reset! (.-cleanup_fn sa) cleanups)
     ;; Return stop function
     (fn []
       (put! stop-ch :stop)
-      (when-let [cleanup @(.-cleanup_fn sa)]
-        (cleanup)))))
+      (doseq [f @(.-cleanup_fn sa)] (f)))))
 
 ;; ---------------------------------------------------------------------------
 ;; Public API
@@ -352,5 +357,4 @@
   "Stop the sync loop and clean up. Does not close the IDB connection."
   [sa]
   (put! (.-stop_ch sa) :stop)
-  (when-let [cleanup @(.-cleanup_fn sa)]
-    (cleanup)))
+  (doseq [f @(.-cleanup_fn sa)] (f)))