Member invites, gallery resync, content and operator docs

- Invite flow: admin issue/revoke on join requests, public single-use
  claim links (hash-only tokens, 7-day expiry, no state reveal), claim
  creates the Member account and marks the request accepted
- Gallery: admin status page plus resync endpoint; sync refreshes
  thumbnails whose content changed; tools/gallery contract and checker
- Docs: public content page, admin-only operator runbook, api.md
  invite/gallery sections, all routes in the live API docs, docs
  reachability gate test
- Screenshots cover the new pages; version 1.0.18

molodetz/templates/admin/gallery.html

@@ -0,0 +1,18 @@
+<!-- retoor <retoor@molodetz.nl> -->
+{% extends "admin/base_admin.html" %}
+{% block admin_content %}
+<h1>Gallery</h1>
+<div class="stat-cards">
+  <div class="stat-card"><span class="value">{{ flyers }}</span><span class="name">Flyers</span></div>
+  <div class="stat-card"><span class="value">{{ memes }}</span><span class="name">Memes</span></div>
+</div>
+{% if missing %}
+<div class="notice error" role="alert"><p>Missing source files:</p><ul>{% for filename in missing %}<li><code>{{ filename }}</code></li>{% endfor %}</ul></div>
+{% else %}
+<p class="muted">All catalogued source files are present.</p>
+{% endif %}
+<form method="post" action="/admin/gallery/resync">
+  <div class="actions"><button class="button" type="submit" data-loading>Resync gallery</button></div>
+</form>
+<p class="muted">Resync re-reads the source files, refreshes changed thumbnails and retires removed entries. New works arrive through deploy first.</p>
+{% endblock %}