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

frontend/src/components/Modal.vue

@@ -1,7 +1,10 @@
 <template>
   <div class="dialog-overlay" @click="$emit('close')">
-    <div ref="dialog" :class="['modal-panel', panelClass]" @keydown="handleDialogKeydown" @click.stop>
-      <slot />
+    <div class="modal-wrap">
+      <div ref="dialog" :class="['modal-panel', panelClass]" @keydown="handleDialogKeydown" @click.stop>
+        <slot />
+      </div>
+      <slot name="attached" />
     </div>
   </div>
 </template>