synapse_port_db: foreign key violation on table refresh_tokens (possible regression?)

### Description

Hello,

Trying to port from sqlite to postgre fails with error:
`twisted.internet.defer.FirstError: FirstError[#41, [Failure instance: Traceback: : insert or update on table "refresh_tokens" violates foreign key constraint "refresh_tokens_next_token_id_fkey"
DETAIL: Key (next_token_id)=(70) is not present in table "refresh_tokens".`

All I could find was issue #8652, fixed by #8730.
Note that attempting a second time, the target DB is bigger at the end of the 2nd attempt than at the end of the 1st, showing the 2nd attempt was able to go further.

### Steps to reproduce

- port from sqlite to a new postgresql database: `sudo synapse_port_db --sqlite-database /var/lib/matrix-synapse/homeserver.db --postgres-config /etc/matrix-synapse/homeserver.yaml --curses`
- process fails after some time with the error mentioned

### Homeserver

matrix.lorteau.fr

### Synapse Version

1.141.0

### Installation Method

Debian packages from packages.matrix.org

### Database

Trying to migrate from sqlite to a single postgresql server on a different host. I don't know the version of the db. I haven't attempted such port before. No backup was restored (but see note in platform below)

### Workers

Single process

### Platform

Synapse was installed using the packages.matrix.org repository and runs on Ubuntu 24.04.3 LTS, inside a VM dedicated to it.
As part of moving away from testing mode the VM was moved to a different host. It is possible that during the life of the VM at some point a snapshot was restored (hence my note on backups). I honestly can't recall.
I have been in "testing" mode a while and my .db file reached 15GB. This is a very much a little home lab setup.

### Configuration

Vanilla install + mautrix-discord + mautrix-gmessages + mautrix-whatsapp.

### Relevant log output

[synapse_port_db.log](https://github.com/user-attachments/files/23377195/synapse_port_db.log)

### Anything else that would be useful to know?

Can't think of any.