`push_rules_stream` table getting huge

### Description

Hello,

I'm managing a (federated) synapse server with 40 users and my `push_rules_stream` table weighs 16GB. It also seems to be increasing but I'm not sure. That table doesn't have any date column so I can't tell how old are most rows but I noticed this:

```
> SELECT 'redacted' AS user_id, count(*) FROM push_rules_stream GROUP BY user_id;

"redacted" 5
"redacted" 634312
"redacted" 178
"redacted" 83
"redacted" 19
"redacted" 1187516
"redacted" 19
"redacted" 99
"redacted" 114
"redacted" 32
"redacted" 22
"redacted" 47
"redacted" 28
"redacted" 3
"redacted" 42
"redacted" 67
"redacted" 63
"redacted" 8
"redacted" 4
"redacted" 6
"redacted" 76
"redacted" 62
"redacted" 27
"redacted" 265
"redacted" 131
"redacted" 40
"redacted" 1
"redacted" 11752101
"redacted" 24
"redacted" 29792667
"redacted" 12805107
"redacted" 66
"redacted" 74
"redacted" 8
"redacted" 124
"redacted" 46
"redacted" 20795959
"redacted" 51
"redacted" 96
"redacted" 77
```

In other words, only a handful of users have produced almost 100% of the rows. Maybe this is triggered by an unexpected behaviour in the clients but it also really looks like a bug in the server, in that they should probably be cleared out after some time. Note that even the events *were* cleared out, because of how postgresql's autovacuuming works, and depending on the pattern with which the events occur over time, autovacuuming might not even help or might need to be tuned. That requires some non-trivial DBA knowledge.

I've noticed these tickets: https://github.com/element-hq/synapse/issues/13456 and https://github.com/element-hq/synapse/issues/5888 but they haven't move in a while.

Until this is fixed, is there some way I can identify and remove unneeded rows from that table ?

BTW, my `state_groups_state` table is also huge (11GB) and I have a few more big ones. There are some open issues already about some of them but they don't seem to have much activity.

### Steps to reproduce

N/A

### Homeserver

my own homeserver

### Synapse Version

1.138.0

### Installation Method

Debian packages from packages.matrix.org

### Database

Using PostgreSQL. Single server.

### Workers

Single process

### Platform

Debian 12

### Configuration

_No response_

### Relevant log output

```shell
N/A
```

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

_No response_

Start by inspecting the schema and write paths for the push_rules_stream table, then read related issues 13456 and 5888. The issue names no source files or tests; done should mean an agreed safe cleanup or retention approach that prevents unbounded growth, with the PostgreSQL impact and regression coverage established.

Written by the indexing model from the issue text.