HTTP API: server, credential and the list of chats (closes #4)

The bot now serves an HTTP API on `PORT` (default 8080) beside the chat client, whose WebSocket stays on 127.0.0.1 inside the container. Every request needs `Authorization: Bearer` with the credential from the file named by `API_TOKEN_FILE`, compared in constant time; with no credential configured every request is refused, `OPTIONS *` included. `GET /api/v1/chats` lists the bot's chats. Responses carry the security headers from the repository policies; bodies, requests and the server are time- and size-bounded. The chat client stops only after the API has finished its requests.

Disclosures: `contact_deleted` is an extra field; 404 and 405 answer in JSON; requests net/http cannot parse are refused by net/http without the security headers; three gosec findings are suppressed as false positives.

Model: opus-5-5

Dockerfile

@@ -119,4 +119,8 @@ USER simplexcalc
 
 ENV DATA_DIR=/var/lib/simplexcalc
 
+# The API, on its default PORT. The chat client's WebSocket on 5225 is
+# not exposed: it has no authentication.
+EXPOSE 8080
+
 CMD ["/app/simplexcalc", "run"]