Simplify battery management: bulk add, device-level auto-install, mass operations

- Replace single-battery add form with bulk add (brand + count, auto-generated labels)
- Add device-level install form: specify brand+qty pairs, system autoselects available batteries
- Add bulk actions on dashboard: retire, delete, unassign, change brand (checkbox multi-select)
- Keep per-battery assign route for special cases (e.g. known low-capacity battery)
- Remove unique constraint on Battery.label (labels are now auto-generated)
- Add *.snapshot to .gitignore for DB snapshot files
- Rewrite tests: 35 passing (11 new tests for bulk-add, device-install, bulk-actions)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/device_detail.html

@@ -25,6 +25,24 @@
   </table>
 </div>
 
+<div class="card">
+  <h2>Install Batteries</h2>
+  {% set free_slots = device.battery_slots - device.installed_count() %}
+  <p class="text-muted" style="margin-bottom:0.75rem;">{{ free_slots }} slot(s) free</p>
+  <form method="post" action="{{ url_for('device_install', device_id=device.id) }}">
+    <div style="display:grid;grid-template-columns:1fr auto;gap:0.4rem;max-width:360px;align-items:center;margin-bottom:0.75rem;">
+      <span style="font-weight:600;font-size:0.85rem;color:#64748b;">Brand</span>
+      <span style="font-weight:600;font-size:0.85rem;color:#64748b;">Qty</span>
+      {% for _ in range(4) %}
+      <input type="text" name="brand[]" placeholder="e.g. Eneloop" style="padding:0.3rem 0.5rem;">
+      <input type="number" name="qty[]" value="0" min="0"
+             style="padding:0.3rem 0.5rem;width:4rem;text-align:center;">
+      {% endfor %}
+    </div>
+    <button class="btn btn-primary" type="submit">Install</button>
+  </form>
+</div>
+
 <div class="card">
   <h2>Installed Batteries</h2>
   {% set installed = device.batteries | selectattr('status', 'eq', 'installed') | list %}