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/gate.html

@@ -0,0 +1,28 @@
+{{define "bodyclass"}}page-gate{{end}}
+{{define "content"}}
+{{- /* A made-up page shape to blur behind the prompt. None of the real
+       site is sent until the password is entered. */}}
+<div class="gate-bg" aria-hidden="true">
+  <div class="container">
+    <div class="gate-sk-profile"><span class="gate-sk-avatar"></span><span class="gate-sk-lines"><i></i><i></i></span></div>
+    <div class="gate-sk-shelf"><i class="gate-sk-heading"></i><div class="gate-sk-row"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></div></div>
+    <div class="gate-sk-shelf"><i class="gate-sk-heading"></i><div class="gate-sk-row"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></div></div>
+    <div class="gate-sk-shelf"><i class="gate-sk-heading"></i><div class="gate-sk-row"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></div></div>
+  </div>
+</div>
+<section class="auth-card gate-card">
+  <div class="auth-mark">{{icon "lock"}}</div>
+  <h1>{{.Site.Title}}</h1>
+  <p class="muted">This collection is private. Enter the password to see it.</p>
+  {{with .Data.Error}}<p class="alert alert--error" role="alert">{{icon "alert"}}{{.}}</p>{{end}}
+  <form method="post" action="/unlock" class="stack">
+    <input type="hidden" name="next" value="{{.Data.Next}}">
+    <label class="field">
+      <span class="field-label">Password</span>
+      <input name="password" type="password" autocomplete="current-password" required autofocus>
+    </label>
+    <button class="btn btn--primary btn--block" type="submit">Enter</button>
+  </form>
+  <a class="auth-back" href="/login?next=/admin">{{icon "user"}}Owner sign in</a>
+</section>
+{{end}}