Server: quiet hours (no wake overnight), zero firmware changes needed

Purely a server-side decision: GET /frame/config hands back a longer
refresh_interval_s while quiet hours are in effect (exactly the seconds
until they end), and clamps the normal interval so the device's next
wake lands at the boundary instead of wandering into the window, when
outside it but approaching. A device already mid-sleep when quiet hours
begin can still land one wake inside the window -- unavoidable without
touching the firmware, since it has no wall-clock awareness -- but from
that wake on it sleeps straight through to the end.

Window is "HH:MM"-"HH:MM", wrap-past-midnight aware (e.g. 22:00-07:00),
in the server's local timezone -- added tzdata to the Dockerfile since
python:3.12-slim doesn't include it and TZ would otherwise silently
resolve to nothing and fall back to UTC.

Also fixed the "overdue" device-status check to account for quiet hours:
without this it would falsely flag a device sleeping through a long
quiet window as unreachable.

server/app/templates/index.html

@@ -99,6 +99,20 @@
         <input type="checkbox" id="smart_crop_faces" {% if cfg.smart_crop_faces %}checked{% endif %}>
         <label for="smart_crop_faces">Center faces in crop</label>
       </div>
+      <div class="checkbox-row">
+        <input type="checkbox" id="quiet_hours_enabled" {% if cfg.quiet_hours_enabled %}checked{% endif %}>
+        <label for="quiet_hours_enabled">Quiet hours (don't wake overnight)</label>
+      </div>
+      <label>Quiet hours start
+        <input type="time" id="quiet_hours_start" value="{{ cfg.quiet_hours_start }}">
+      </label>
+      <label>Quiet hours end
+        <input type="time" id="quiet_hours_end" value="{{ cfg.quiet_hours_end }}">
+      </label>
+      <p class="sub">Uses the server's local timezone (see <code>TZ</code> in
+        <code>docker-compose.yml.example</code>). The device may still
+        wake once right at the start of quiet hours -- it can't know
+        ahead of time -- but goes right back to sleep until they end.</p>
       <label>Upcoming photos to show
         <select id="queue_target_len">
           {% for n in [5, 10, 15, 20, 25, 30, 40, 50] %}
@@ -151,6 +165,9 @@
         refresh_interval_s: String(minutes * 60),
         smart_crop_faces: String(document.getElementById('smart_crop_faces').checked),
         queue_target_len: document.getElementById('queue_target_len').value,
+        quiet_hours_enabled: String(document.getElementById('quiet_hours_enabled').checked),
+        quiet_hours_start: document.getElementById('quiet_hours_start').value || '22:00',
+        quiet_hours_end: document.getElementById('quiet_hours_end').value || '07:00',
       });
       const resp = await fetch('/api/config', {
         method: 'POST',