06/16 Phase 6 foundation: admin blueprint, audit log, user management

Adds the admin backend foundation: AuditLog + Setting models/migration,
an admin-only dashboard with KPI cards, and user management (search,
ban/suspend/activate, tier override, trust adjust) with audit logging
on every write action. Smoke suite extended to 64 checks.

app/templates/base.html

@@ -24,6 +24,9 @@
             {% if unread_count %}<span class="badge-count">{{ unread_count }}</span>{% endif %}
           </a>
           <a href="{{ url_for('payments.my_billing') }}">{{ _('Billing') }}</a>
+          {% if current_user.is_admin %}
+            <a href="{{ url_for('admin.dashboard') }}">{{ _('Admin') }}</a>
+          {% endif %}
           <span class="hi">{{ current_user.display_name }}</span>
           <a href="{{ url_for('auth.logout') }}">{{ _('Sign out') }}</a>
         {% else %}