feat: make Updates discovery atomic (Closes #753)

src/main.py

@@ -3592,6 +3592,21 @@ async def notification_page(page: int = Query(default=1, ge=1)) -> JSONResponse:
     return JSONResponse(result)
 
 
+@app.get("/api/v1/notifications/snapshot")
+async def notification_snapshot() -> JSONResponse:
+    try:
+        result = await gitea_proxy.unread_notification_snapshot(
+            deadline_seconds=NOTIFICATION_PAGE_TIMEOUT_SECONDS
+        )
+    except Exception:
+        return JSONResponse(
+            {"error": "Complete unread updates are temporarily unavailable. Please retry."},
+            status_code=503,
+            headers={"Retry-After": "1"},
+        )
+    return JSONResponse(result)
+
+
 @app.get("/api/v1/notifications/{thread_id}")
 async def notification_thread_detail(
     thread_id: int = PathParam(gt=0),