Store the user-directory temp position as BIGINT

The `populate_user_directory` background update stored its stream position in a
`TEXT` column (`_temp_populate_user_directory_position.position`) but wrote an
int into it and read it back into the `BIGINT`
`user_directory_stream_pos.stream_id` — the one place in the storage layer that
bound an int to a text column (and a numeric string back to an int column). This
only worked because psycopg2 coerces such literals; the native Rust driver binds
typed parameters and rejects the mismatch.

Fix it at the source: make the temp column `BIGINT`, matching the value it holds
(and `update_user_directory_stream_pos`'s `int` type hint, which the `TEXT`
column had been quietly violating). The temp table is created and dropped within
the background update, so there's no migration.

Tested: user_directory storage + handler tests pass on all three backends
(psycopg2, sqlite, Rust).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W3G4M92AmwSSZCbmtMJU3d

synapse/storage/databases/main/user_directory.py

@@ -125,6 +125,10 @@ class UserDirectoryBackgroundUpdateStore(StateDeltasStore):
                 f"CREATE INDEX IF NOT EXISTS {TEMP_TABLE}_rooms_evs ON {TEMP_TABLE}_rooms (events)"
             )
 
+            # NB: the `position` column used to be TEXT; `IF NOT EXISTS` keeps
+            # a table made by an older release, which the schema delta
+            # 94/05_temp_user_dir_position_bigint.sql.postgres upgrades in
+            # place.
             sql = f"""
                 CREATE TABLE IF NOT EXISTS {TEMP_TABLE}_position (
                     position BIGINT NOT NULL

synapse/storage/schema/main/delta/94/05_temp_user_dir_position_bigint.sql.postgres

@@ -0,0 +1,21 @@
+--
+-- 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>.
+
+-- The user-directory rebuild's staging table changed its `position` column
+-- from TEXT to BIGINT, but the table is created with IF NOT EXISTS and only
+-- dropped when the populate_user_directory background update completes, so a
+-- server upgrading mid-rebuild keeps the old TEXT column — which the native
+-- Rust driver's strictly-typed binding can neither read nor write. Upgrade it
+-- in place; a no-op for servers without the table (i.e. almost all of them).
+ALTER TABLE IF EXISTS _temp_populate_user_directory_position
+    ALTER COLUMN position TYPE BIGINT USING position::bigint;