Add bulk install-in-device from dashboard and unretire action

- Dashboard bulk toolbar: select batteries, pick a device, click 'Install
  in device'; confirms before moving already-installed batteries; enforces
  slot capacity and warns on brand mix
- Battery detail: 'Unretire Battery' button replaces 'Retire Battery' when
  battery is retired, restoring it to available status
- Tests: 3 new bulk-install-device tests (capacity block, move, success);
  42 total passing

templates/battery_detail.html

@@ -185,6 +185,10 @@
       <form method="post" action="{{ url_for('battery_retire', battery_id=battery.id) }}">
         <button class="btn btn-secondary" type="submit">Retire Battery</button>
       </form>
+    {% else %}
+      <form method="post" action="{{ url_for('battery_unretire', battery_id=battery.id) }}">
+        <button class="btn btn-secondary" type="submit">Unretire Battery</button>
+      </form>
     {% endif %}
 
     <a class="btn btn-danger" href="{{ url_for('battery_delete', battery_id=battery.id) }}">Delete Battery</a>