fix(backend): a bulk deletion is announced in chunks that fit the notify payload limit

backend/src/utils/events.py

@@ -4,6 +4,7 @@ from typing import Literal
 import asyncpg
 
 BG_EVENTS_CHANNEL = "bg_events"
+MESSAGE_IDS_PER_EVENT = 500
 
 EventKind = Literal["message", "edit", "delete", "reaction", "presence", "receipt"]
 
@@ -22,8 +23,15 @@ async def notify_bg_event(  # noqa: PLR0913
         payload["chat_id"] = chat_id
     if message_id is not None:
         payload["message_id"] = message_id
-    if message_ids is not None:
-        payload["message_ids"] = message_ids
+    if message_ids is None:
+        await _notify(pool, payload)
+        return
+    for start in range(0, len(message_ids), MESSAGE_IDS_PER_EVENT):
+        chunk = message_ids[start : start + MESSAGE_IDS_PER_EVENT]
+        await _notify(pool, {**payload, "message_ids": chunk})
+
+
+async def _notify(pool: asyncpg.Pool, payload: dict[str, object]) -> None:
     await pool.execute(
         "SELECT pg_notify($1, $2)", BG_EVENTS_CHANNEL, json.dumps(payload)
     )