Stop allowing cross-origin requests (closes #261)

The CORS middleware let every origin make credentialed requests, so a
page on another subdomain of the server's domain could read and act as
the signed-in user. Nothing needs cross-origin access, so the
middleware and the go-chi/cors dependency go. A test checks that a
request from another origin and a preflight get no
Access-Control-Allow-* headers.

Model: opus-5-5

go.mod

@@ -7,7 +7,6 @@ require (
 	github.com/gdamore/tcell/v2 v2.13.8
 	github.com/getsentry/sentry-go v0.42.0
 	github.com/go-chi/chi/v5 v5.2.1
-	github.com/go-chi/cors v1.2.2
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
 	github.com/prometheus/client_golang v1.23.2