feat(auth): add an admin-managed login allowlist

Lets the admin restrict which pubkeys may log in, enforced server-side
at /api/auth/login before a session is issued. Disabled by default;
the admin and the bootstrap (no-admin-claimed-yet) case always pass.
Manageable via the existing settings UI/API (npub or hex, one per line).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -1,5 +1,6 @@
 <script setup lang="ts">
 import { onMounted, reactive, ref } from 'vue';
+import { nip19 } from 'nostr-tools';
 import { api } from '../lib/api';
 import { useAuthStore } from '../stores/auth';
 
@@ -8,28 +9,64 @@ interface Settings {
   relays: string[];
   public_url: string;
   admin_pubkey: string | null;
+  login_allowlist_enabled: boolean;
+  login_allowlist: string[];
 }
 
 const auth = useAuthStore();
-const form = reactive({ blossom_url: '', relays: '', public_url: '' });
+const form = reactive({
+  blossom_url: '',
+  relays: '',
+  public_url: '',
+  login_allowlist_enabled: false,
+  login_allowlist: '',
+});
 const saved = ref(false);
 const error = ref('');
 
+/** Accepts npub or raw hex, one per line; returns lowercase hex. Throws on anything invalid. */
+function parseAllowlist(text: string): string[] {
+  return text
+    .split('\n')
+    .map((l) => l.trim())
+    .filter(Boolean)
+    .map((line) => {
+      if (line.startsWith('npub1')) {
+        const decoded = nip19.decode(line);
+        if (decoded.type !== 'npub') throw new Error(`not an npub: ${line}`);
+        return decoded.data;
+      }
+      if (!/^[0-9a-f]{64}$/i.test(line)) throw new Error(`not a valid npub or hex pubkey: ${line}`);
+      return line.toLowerCase();
+    });
+}
+
 onMounted(async () => {
   const s = await api.get<Settings>('/api/settings');
   form.blossom_url = s.blossom_url;
   form.relays = s.relays.join('\n');
   form.public_url = s.public_url;
+  form.login_allowlist_enabled = s.login_allowlist_enabled;
+  form.login_allowlist = s.login_allowlist.map((pk) => nip19.npubEncode(pk)).join('\n');
 });
 
 async function save() {
   error.value = '';
   saved.value = false;
+  let allowlist: string[];
+  try {
+    allowlist = parseAllowlist(form.login_allowlist);
+  } catch (err) {
+    error.value = (err as Error).message;
+    return;
+  }
   try {
     await api.put('/api/settings', {
       blossom_url: form.blossom_url,
       relays: form.relays.split('\n').map((r) => r.trim()).filter(Boolean),
       public_url: form.public_url,
+      login_allowlist_enabled: form.login_allowlist_enabled,
+      login_allowlist: allowlist,
     });
     saved.value = true;
   } catch (err) {
@@ -62,6 +99,24 @@ async function save() {
         <input id="set-public" v-model="form.public_url" class="input" type="url" required />
         <p class="mt-1 text-xs text-white/30">Used in RSS feed links. Must be reachable by podcast apps.</p>
       </div>
+      <div class="border-t border-white/10 pt-4">
+        <label class="flex items-center gap-2 text-sm">
+          <input v-model="form.login_allowlist_enabled" type="checkbox" />
+          Restrict logins to an allowlist
+        </label>
+        <p class="mt-1 text-xs text-white/30">
+          When enabled, only the admin and pubkeys listed below can log in. Everyone else is
+          rejected at login (existing sessions aren't revoked).
+        </p>
+        <textarea
+          id="set-allowlist"
+          v-model="form.login_allowlist"
+          class="input mt-2 font-mono text-sm"
+          rows="6"
+          placeholder="npub1... (one per line, npub or hex)"
+          :disabled="!form.login_allowlist_enabled"
+        />
+      </div>
       <p v-if="!auth.isAdmin" class="rounded-lg bg-amber-500/20 border border-amber-500/40 p-3 text-sm text-amber-200">
         Only the admin (first account to log in) can change settings.
       </p>