Add an optional site password for visitors

Settings → Privacy can set a password for the whole public site. Until a
visitor enters it, every public page shows a prompt over a blurred
made-up page, and images, preview clips and exports return 401; nothing
real is sent. Unlocking sets a 90-day cookie derived from the password
hash, so changing the password asks everyone again. The owner is never
asked while signed in. Wrong guesses are rate limited, and a locked site
sends noindex.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/templates/pages/admin_settings.html

@@ -11,6 +11,7 @@
     <nav class="settings-nav" aria-label="Settings sections">
       <a href="#profile">{{icon "user"}}Profile</a>
       <a href="#appearance">{{icon "palette"}}Appearance</a>
+      <a href="#privacy">{{icon "eye-off"}}Privacy</a>
       <a href="#integrations">{{icon "plug"}}Integrations</a>
       <a href="#backups">{{icon "archive"}}Backups</a>
       <a href="#account">{{icon "lock"}}Account</a>