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: #4

docs/proxy/traefik.md

@@ -85,13 +85,14 @@ authResponseHeaders:
   - Remote-Role-Name
   - Remote-Session-Expires
   - Remote-Credential
+  - Remote-Public
 ```
 
 ## Proxying `/auth/` to Paskia
 
 The `/auth/` router above forwards all authentication UI, API, and WebSocket traffic to Paskia. Because this router does **not** use the `paskia-auth` middleware, users can reach the login page and profile UI without being authenticated first. Traefik handles WebSocket upgrades automatically when the client requests them.
 
-If you are using a dedicated authentication host instead of `/auth/`, create a separate router for `auth.example.com` pointing to the Paskia service and start Paskia with `--auth-host auth.example.com`.
+If you are using a dedicated authentication host instead of `/auth/`, create a separate router for `auth.example.com` pointing to the Paskia service and set the domain's auth host in the admin panel's Domains section.
 
 ## Adjusting requirements
 
@@ -119,6 +120,17 @@ labels:
   - "traefik.http.middlewares.paskia-auth.forwardauth.authRequestHeaders=Host,Cookie,Accept,X-Forwarded-Method,X-Forwarded-Uri"
 ```
 
+## Public access
+
+For routes where anonymous visitors are allowed but logged-in users should still be identified, add `public=1` to the middleware `address`:
+
+```yaml
+address: "http://localhost:4401/auth/api/forward?public=1"
+address: "http://localhost:4401/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 — copied by `authResponseHeadersRegex: "^Remote-"` — 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
 
 - By default ForwardAuth sends a request without the original body. If you need to forward the body for logging/validation, set `forwardBody: true` and a sensible `maxBodySize`, but for Paskia this is not required.