MultiSite: one instance serves authentication across many domains (#4)

- Serve multiple domains (RP IDs) from one instance: host-based dispatch,
  per-domain credentials and sessions, domains managed at runtime in the
  admin UI — previously one RP per instance
- Cross-domain sign-in via Related Origin Requests: per-domain related-origins
  list with a served .well-known/webauthn document
- Explicit per-domain origin lists with shell-glob wildcards (**. for apex +
  any subdomain depth, *. for one level), editable in the admin UI with
  validation and self-lockout guards
- Per-domain auth hosts: the account/admin UI can live on a different host
  per domain, no longer confined to subdomains of a single RP
- CLI: 'paskia init <rp-id [rp-name]' initializes or adds a domain to an
  existing database; 'paskia migrate' converts legacy databases

BREAKING CHANGES (v2.0):
- Database schema: config is now per-domain and credentials/sessions carry
  an rp_id — existing databases must be converted with 'paskia migrate'
- Origins are now explicit: main implicitly allowed every subdomain of the
  RP; configure '**.' origins to reproduce that behavior
- CLI: the flat '--rp-id/--rp-name/--origin/--auth/--save' flags are
  replaced by the 'init' and 'migrate' subcommandsReviewed-on: https://git.zi.fi/LeoVasanko/paskia/pulls/4

docs/proxy/envoy.md

@@ -161,7 +161,18 @@ See [perm argument](../api/perm.md) and [max_age argument](../api/max-age.md) fo
 
 ## WebSocket support for `/auth/`
 
-If you use a dedicated authentication host (`--auth-host`), route `auth.example.com` to the Paskia cluster and you do not need the `/auth/` bypass above. Otherwise, make sure the `/auth/` route keeps the `Upgrade` and `Connection` headers so passkey WebSocket endpoints work. The default Envoy router handles `Upgrade` headers when the client requests them.
+If you use a dedicated authentication host (the domain's auth-host setting), route `auth.example.com` to the Paskia cluster and you do not need the `/auth/` bypass above. Otherwise, make sure the `/auth/` route keeps the `Upgrade` and `Connection` headers so passkey WebSocket endpoints work. The default Envoy router handles `Upgrade` headers when the client requests them.
+
+## Public access
+
+For routes where anonymous visitors are allowed but logged-in users should still be identified, add `public=1` to `path_override` (globally or per route):
+
+```yaml
+path_override: "/auth/api/forward?public=1"
+path_override: "/auth/api/forward?public=1&perm=myapp:reports"
+```
+
+The auth check then always returns 204 (except reauth with `max_age`, which still returns the 401 auth flow), and the `Remote-Public` header — matched by the `prefix: Remote-` rule in `allowed_upstream_headers` — marks each request as `anonymous`, `forbidden` or `authenticated`. The backend always runs and must check `Remote-Public` before treating the request as authorized. See [public access](../api/forward.md#public-access) and [trusted headers](../Headers.md#public-access).
 
 ## Notes