Add destination_room_sticky_events_backlog table

synapse/storage/schema/main/delta/93/05_destination_room_sticky_events.sql

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+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2026 Element Creations, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+
+-- Tracks rooms that have outstanding sticky events that still need to be
+-- sent to a destination (remote homeserver).
+--
+-- Essentially a queue of unsent sticky events, but with a compact storage
+-- representation that only needs one row per (destination, room) pair.
+--
+-- Each row means: in the given room and for the given destination,
+-- we still need to send sticky events with `stream_id` at or higher than
+-- `sticky_events_stream_position`.
+--
+-- Due to sticky event expiration and event deletion, rows in this table are
+-- hints that there *may* be sticky events to send, but not a guarantee that
+-- there actually are.
+CREATE TABLE destination_room_sticky_events_backlog (
+    -- Server name of the remote homeserver.
+    destination TEXT NOT NULL,
+
+    -- Room ID in which sticky events have been missed.
+    room_id TEXT NOT NULL
+        -- Only track this information for rooms we know about;
+        -- if we delete a room locally then also delete the tracking info.
+        REFERENCES rooms(room_id) ON DELETE CASCADE,
+
+    -- Position in the sticky events stream, corresponding to the
+    -- `sticky_events.stream_id` of the first sticky event that
+    -- has yet to be sent.
+    --
+    -- Because sticky events must be sent in order (as per MSC4354),
+    -- all subsequent sticky events for the same room with higher
+    -- `stream_id`s are also unsent.
+    --
+    -- Not a foreign key because we must still support expiration of sticky events.
+    sticky_events_stream_position INTEGER NOT NULL,
+
+    -- It's enough to track one position per (destination, room_id) pair.
+    PRIMARY KEY (destination, room_id)
+);
+
+-- Should have this index to make `room_id` foreign key constraint efficient,
+-- as well as for cleanup per room.
+CREATE INDEX destination_room_unsent_sticky_events_room_id ON destination_room_unsent_sticky_events (room_id);