Name the cause on the page for a refused form (closes #358)

The CSRF middleware answered a refused login or generator form with the
library's bare text, such as "Forbidden - origin invalid". It now answers
403 with a page that names the cause and what to do, and logs a warning
with the request ID and the library's reason. A page opened over plain
HTTP with debug off and a form from another host both fail the origin
check; the page tells them apart by the host in the Origin or Referer
header.

Model: opus-5-5

internal/handlers/handlers.go

@@ -56,14 +56,16 @@ type Handlers struct {
 
 // New creates a new Handlers instance.
 func New(_ fx.Lifecycle, params Params) *Handlers {
+	log := params.Logger.Get()
+
 	return &Handlers{
-		log:              params.Logger.Get(),
+		log:              log,
 		hc:               params.Healthcheck,
 		config:           params.Config,
 		imgSvc:           params.ImageService,
 		sessMgr:          params.SessionManager,
 		encGen:           params.EncryptedURLGenerator,
-		csrfProtect:      newCSRFProtect(params.Keys.CSRFKey, params.Config.Debug),
+		csrfProtect:      newCSRFProtect(params.Keys.CSRFKey, params.Config.Debug, log),
 		signer:           signature.New(params.Config.SigningKey),
 		allowlist:        allowlist.New(params.Config.AllowlistHosts),
 		refererBlocklist: allowlist.New(params.Config.RefererBlocklist),