cpu usage skyrocketing for no apparent reason

### Background information

- **Dendrite version or git SHA**: 0.14.1+40bef6a
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: yes

### Description

Hi, since few days ago my instance cpu usage is skyrocketing with one core at 100% constantly.
Thanks to docker I'm able to mitigate this by limiting the cpus amount I provide to the container.
The weird thing is, afaik, I didn't change the config recently. Also, presence feature is disabled since the beginning (both in/outbound set to false). I have no clue on what could be the root cause.
Restarting/recreating the container (but preserving any volume config and the db) does not help.
No error or warns in the logs.
All I can see is a decent pressure of http calls to
```
/_matrix/client/v3/sync?filter=1&timeout=30000&org.matrix.msc1234.use_state_after=true&set_presence=unavailable&since=blabla
```
Any ideas?

Begin with the /_matrix/client/v3/sync request pattern, the Postgres and Docker setup, and the available logs; compare CPU behavior with presence disabled. Done means identifying a reproducible cause for the sustained single-core usage and documenting a verified fix or mitigation.

Written by the indexing model from the issue text.