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/haproxy.md

@@ -123,4 +123,4 @@ The `Remote-*` success-headers glob already copies the `Remote-Public` header th
 - The Lua script strips the request body from the auth subrequest, so Paskia's `/auth/api/forward` will only see the headers.
 - HAProxy variables are limited to alphanumeric characters, dots, and underscores, but the script already normalizes header names for you (e.g. `Remote-User` becomes `req.auth_response_header.remote_user`). The `Remote-*` glob pattern in the success-headers argument handles this automatically.
 - The auth backend must be reachable without TLS. If you need TLS to Paskia, run a local TCP forwarder or use HAProxy's Lua HTTP support directly (not covered by this script).
-- If you use a dedicated authentication host (`--auth-host`), route `auth.example.com` to the Paskia backend and start Paskia with `--auth-host auth.example.com` instead of exposing `/auth/` on every site.
+- If you use a dedicated authentication host (the domain's auth-host setting), route `auth.example.com` to the Paskia backend instead of exposing `/auth/` on every site.