Require HTTPS for OIDC and public URLs; stop logging token responses

Address review findings on a10b24c:

- F1: the OIDC issuer and the discovered authorization, token and JWKS
  endpoints must be https://; plain http:// is allowed only for the
  loopback hosts localhost, 127.0.0.1 and [::1]. Config rejects a remote
  http:// issuer at startup; auth.New rejects a discovered remote http://
  endpoint, naming it, before it is used.
- F2: a failed code exchange is reduced to a fixed category plus the HTTP
  status; the token endpoint's body and error_description are never logged
  or wrapped into the returned error.
- F3: MEALQUEST_PUBLIC_URL must be https:// unless its host is loopback,
  so the Secure login cookie is always sent back on the callback.

The rule lives once, in internal/secureurl.Check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -43,7 +43,9 @@ make dev
 (which proxies `/api`, `/auth` and `/media` to `:8080`). Open
 http://localhost:5173. Configuration is environment-only and fails fast:
 every variable above is required, and secrets are read only from files named
-by `*_FILE` variables.
+by `*_FILE` variables. `MEALQUEST_PUBLIC_URL`, `MEALQUEST_OIDC_ISSUER` and the
+endpoints the issuer advertises must be `https://`; plain `http://` is
+accepted only for `localhost`, `127.0.0.1` and `[::1]`.
 
 ## Common commands