Ensure `POST /files/content` doesn't hang in api-server

[bisgaard-itis]

• Joel discovered that POST /files/content sometimes hangs in the api-server.
• If the backend doesn't respond in time, the api-server should respond with a 504/502.
• As part of this ticket it would be good to add some logging/tooling to be able to track down the root cause for the hang.