Bulk Dell warranty sync + add-warranty from asset pages

- POST /warranty/sync/dell: auto-detect Dell hardware by service tag (asset
  serial) and create warranties for the ones Dell recognizes, in batches of 100.
  Only looks up assets that lack a dated warranty, so re-runs are cheap and
  non-Dell serials are filtered out by Dell. DellProvider.bulk_lookup batches
  tags over the cached token.
- "Sync Dell" button on the Warranties page with a toast summary.
- WarrantyPanel "Add one / Add-manage" links deep-link to the warranties add
  modal pre-linked to that asset (?addfor=); WarrantiesList opens it prefilled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/WarrantyPanel.vue

@@ -14,12 +14,12 @@
           <span v-if="w.servicetag" class="mono">Tag {{ w.servicetag }}</span>
         </div>
       </div>
-      <router-link to="/warranties" class="warranty-manage">Manage warranties</router-link>
+      <router-link :to="`/warranties?addfor=${assetid}`" class="warranty-manage">Add / manage</router-link>
     </div>
 
     <div v-else class="warranty-empty">
       <span class="muted">No warranty on record.</span>
-      <router-link to="/warranties" class="warranty-manage">Add one</router-link>
+      <router-link :to="`/warranties?addfor=${assetid}`" class="warranty-manage">Add one</router-link>
     </div>
   </div>
 </template>