Add optional battery metadata fields

New nullable columns on Battery: size, chemistry, capacity_mah,
tested_capacity_mah, tested_date, charge_cycles, purchase_date.

Battery detail page shows all populated fields and a full edit form
with select dropdowns for size and chemistry (with Other fallback).
Capacity health % shown in green/orange/red when both nominal and
tested capacity are set. Dashboard gains a Size column.

templates/dashboard.html

@@ -51,6 +51,7 @@
             <th style="width:1.5rem;"><input type="checkbox" id="select-all" title="Select all"></th>
             <th>Label</th>
             <th>Brand</th>
+            <th>Size</th>
             <th>Status</th>
             <th>Assigned To</th>
             <th>Actions</th>
@@ -62,6 +63,7 @@
             <td><input type="checkbox" name="battery_ids" value="{{ b.id }}" class="row-cb"></td>
             <td><a href="{{ url_for('battery_detail', battery_id=b.id) }}"><strong>{{ b.label }}</strong></a></td>
             <td>{{ b.brand }}</td>
+            <td>{{ b.size or '—' }}</td>
             <td>
               <span class="badge badge-{{ b.status }}">{{ b.status|capitalize }}</span>
             </td>
@@ -94,7 +96,7 @@
             </td>
           </tr>
           {% else %}
-          <tr><td colspan="6" class="text-muted" style="text-align:center;padding:1rem;">No batteries found. <a href="{{ url_for('battery_add') }}">Add some.</a></td></tr>
+          <tr><td colspan="7" class="text-muted" style="text-align:center;padding:1rem;">No batteries found. <a href="{{ url_for('battery_add') }}">Add some.</a></td></tr>
           {% endfor %}
         </tbody>
       </table>