Three features: device dropdown filter, charge log history, unassign-all

- Device dropdowns (quick-assign, bulk install, assign page) now only show
  devices with free slots; full devices are excluded entirely
- New ChargeLog model tracks charge dates with optional cycle increment;
  battery detail page gets a Charge History card with add/delete rows
- Device list page gets per-device Unassign All button (with confirmation)
  via new POST /device/<id>/unassign-all route

templates/device_list.html

@@ -57,6 +57,12 @@
           </td>
           <td data-label="Actions" style="white-space:nowrap;">
             <a class="btn btn-sm btn-secondary" href="{{ url_for('device_detail', device_id=d.id) }}">View</a>
+            {% if d.installed_count() > 0 %}
+            <form class="inline" method="post" action="{{ url_for('device_unassign_all', device_id=d.id) }}"
+                  onsubmit="return confirm('Unassign all batteries from {{ d.name }}?')">
+              <button class="btn btn-sm btn-warning" type="submit">Unassign All</button>
+            </form>
+            {% endif %}
             <form class="inline" method="post" action="{{ url_for('device_delete', device_id=d.id) }}"
                   onsubmit="return confirm('Delete {{ d.name }}? All installed batteries will be unassigned.');">
               <button class="btn btn-sm btn-danger" type="submit">Delete</button>