[TODO] add middleware

We only have auth middleware. That's pretty weak sauce. Add:

```fs
/src/middleware/
auth.js
moderation.js
chatDisabled.js
rateLimit.js
validate.js
errorHandler.js
```

Start by inspecting the existing /src/middleware/auth.js and the surrounding middleware entry points. Before implementation, define the expected behavior and integration points for moderation, chatDisabled, rateLimit, validate, and errorHandler; done means those six middleware files exist and their requirements are covered by project tests.

Written by the indexing model from the issue text.