Dendrite 0.13.0 onwards causes unrecognized request on server URL input on Element Android

*This issue was originally created by [**@Alexilator**](https://github.com/Alexilator) at .*

### Background information

- **Dendrite version or git SHA**: 0.13.0+
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: yes
- **Client used (if applicable)**: Element Android App

### Description

- **What** is the problem: since Dendrite-monolith 0.13.0 a login with Element Android is not possible
- **Who** is affected: as far as I can tell, everyone using dendrite 0.13.0 onwards and Element Android
- **How** is this bug manifesting: "unrecognized request" message below the custom server URL input field.
- **When** did this first appear: Dendrite-monolith 0.13.0

### Steps to reproduce

- install / upgrade to dendrite-monolith inside docker 0.13.0 or later
- input matrix domain of dendrite-monolith in Element Android App
- error "unrecognized request" will be shown below the input

There is nothing in the container logs regarding the client error.

No source files or tests are named. Reproduce with Dendrite-monolith 0.13.0+ in Docker, PostgreSQL, and Element Android using the listed server-domain steps, then inspect the request handling and container behavior around the unrecognized-request response. Done means Element Android accepts the custom server URL and login succeeds without the error.

Written by the indexing model from the issue text.