Refuse API writes that are not application/json (closes #271)

A page on another origin of the same site could send a text/plain or
form POST with the user's cookie, since a browser sends such a request
without a preflight. RequireJSON on the /api/v1 router answers 415 to a
POST, PUT or DELETE whose Content-Type is not application/json, or that
has none, before the handler runs.

The web client and neoirc-cli already send the header on every request.

Model: opus-5-5

README.md

@@ -157,6 +157,10 @@ either.
 ### Conventions
 
 - Request and response bodies are JSON.
+- Every `POST`, `PUT` and `DELETE` request under `/api/v1/` needs the header
+  `Content-Type: application/json`, even one with no body. A request with
+  another type, or none, gets `415` with
+  `{"error": "content type must be application/json"}`.
 - Endpoints marked "needs auth" need the `neoirc_auth` cookie.
   `POST /api/v1/session` and `POST /api/v1/login` set it. A request without a
   valid cookie gets `401` with `{"error": "not registered", "numeric": 451}`.
@@ -517,7 +521,8 @@ for every HTTP request when metrics are on.
 Every other `GET` path serves the web client. A path that names one of its files
 gets that file; any other path gets its `index.html`. An unknown path under
 `/api/v1/` gets a plain-text `404`. A request whose method the path does not
-take gets `405` with no body.
+take gets `405` with no body. Under `/api/v1/`, a `POST`, `PUT` or `DELETE`
+without `Content-Type: application/json` gets `415` instead.
 
 ### The message envelope
 
@@ -2006,7 +2011,8 @@ and their sessions to end. Then it closes the database.
 - `internal/hashcash/`: stamp checking and minting.
 - `internal/ratelimit/`: the per-address login rate limiter.
 - `internal/middleware/`: finding the client's address, request logging,
-  cross-origin requests, the `Content-Security-Policy` header and metrics.
+  cross-origin requests, the API's `Content-Type` check, the
+  `Content-Security-Policy` header and metrics.
 - `internal/healthcheck/` and `internal/stats/`: the health check and its
   counters.
 - `internal/logger/` and `internal/globals/`: logging setup and build