Keep notification SSE stream alive behind proxies (#1516)

* fix: keep notification SSE stream alive

* Fix PR comments

* fix: cover notification stream error cleanup

---------

Co-authored-by: Konstantin Zolin <zolin_ka@vk.com>
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/web/server/lib/notifications/DOCUMENTATION.md

@@ -26,6 +26,7 @@ This module provides notification message preparation utilities for the web serv
   - `DELETE /api/push/subscribe`
   - `POST /api/push/visibility`
   - `GET /api/push/visibility`
+  - `GET /api/notifications/stream`
   - `GET /api/session-activity`
   - `GET /api/sessions/snapshot`
   - `GET /api/sessions/status`
@@ -86,6 +87,7 @@ This module provides notification message preparation utilities for the web serv
 
 ### Default values
 - `DEFAULT_NOTIFICATION_MESSAGE_MAX_LENGTH`: 250 (default max length for notification text).
+- `NOTIFICATION_SSE_HEARTBEAT_INTERVAL_MS`: 20000 (notification SSE comment heartbeat interval).
 
 ## Settings object format