Consider adding (more?) foreign key constraints

This issue has been migrated from [#12817](https://github.com/matrix-org/synapse/issues/12817).

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**Description:**

Because of an unfortunate event, I ended up on my server with some extra accounts and rooms I wanted to delete. I used the proper (`admin/v1/deactivate`, `admin/v1/rooms` and `admin/v2/rooms`) APIs of Synapse, but it left me with a database that appears inconsistent:

- I had 10 GB of entries in `current_state_delta_stream`, `state_groups_state` and `state_groups`, but the rooms were missing from `room_stats_state`
- half of my rows in `state_group_edges` don't point to `state_groups`

I'd like to clean these up, but it's hard for me because I'm not familiar with the database structure and there are no constraints to help with it.

Adding some constraints would help avoid inconsistent states (cf. #11779) and even work as documentation. If you're worried about the performance impact, they could added in a disabled state.