ui batch: admin tab, time filter, count-up, chevron sync, tidy

1. New /admin route with sub-tabs (Protokoll, Benutzer) for admins.
   Top nav: "Protokoll" dropped, "Admin" added right of Einstellungen.
   /logs and /einstellungen/benutzer issue 301 redirects to the new paths.
   Benutzer is no longer part of Einstellungen sub-nav.
2. User_filters.max_age_hours (migration v6) — new dropdown (1–10 h /
   beliebig) under Einstellungen → Filter; Wohnungen list drops flats
   older than the cutoff by discovered_at.
3. Header shows "aktualisiert vor X s" instead of a countdown. Template
   emits data-counter-up-utc with last_alert_heartbeat; app.js ticks up
   each second. When a scrape runs, the heartbeat updates and the HTMX
   swap resets the counter naturally.
4. Chevron state synced after HTMX swaps: panes preserved via hx-preserve
   keep the user's open/closed state, and the sibling button's .open
   class is re-applied by syncFlatExpandState() on afterSwap — previously
   a scroll-triggered poll would flip the chevron back to closed while
   the pane stayed open.
5. "Final absenden" footer removed from the profile page (functionality
   is unchanged, the switch still sits atop Wohnungen).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/admin.html

@@ -0,0 +1,22 @@
+{#
+  Admin-only landing page. Merges the former /logs tab and the former
+  Benutzer sub-section of Einstellungen into one place, with sub-tabs.
+#}
+{% extends "_layout.html" %}
+{% block content %}
+<section class="card">
+    <nav class="flex flex-wrap border-b border-soft px-4">
+        {% set sections = [('protokoll','Protokoll'), ('benutzer','Benutzer')] %}
+        {% for key, label in sections %}
+        <a href="/admin/{{ key }}"
+           class="tab {% if section == key %}active{% endif %}">{{ label }}</a>
+        {% endfor %}
+    </nav>
+
+    <div class="p-5">
+        {% if section == 'protokoll' %}{% include "_admin_logs.html" %}
+        {% elif section == 'benutzer' %}{% include "_settings_users.html" %}
+        {% endif %}
+    </div>
+</section>
+{% endblock %}