feat: robust ML publish with pre-flight, preview, validation, async

- Add /inventory-check endpoint for local pre-flight validation
- Add /listings/validate endpoint using ML /items/validate API
- Add /categories/<id>/attributes endpoint for required attrs
- Add /listings/async + polling for background publishing via Celery
- Editable preview: title (0/60 counter), price, stock per item
- Pre-flight checks: image, stock, price, duplicate detection
- Image upload directly from publish modal (uses existing /items/<id>/image)
- Dynamic required attributes form based on selected ML category
- Frontend: validate button, async polling with progress, detailed error display
- Backend: build_item_payload supports custom_title, extra_attributes

pos/static/css/inventory.css

@@ -1333,6 +1333,73 @@
     /* History table inside modal */
     .inv-modal .data-table { width: 100%; }
 
+    /* ─── MercadoLibre Publish Modal Enhancements ────────────────────────── */
+    .meli-preview-card {
+      display: grid;
+      grid-template-columns: 56px 1fr auto auto auto;
+      gap: var(--space-3);
+      align-items: center;
+      padding: var(--space-3);
+      border: 1px solid var(--color-border);
+      border-radius: var(--radius-md);
+      margin-bottom: var(--space-2);
+      background: var(--color-surface-1);
+    }
+    .meli-preview-card img {
+      width: 56px; height: 56px; object-fit: cover; border-radius: var(--radius-sm);
+      background: var(--color-surface-2);
+    }
+    .meli-preview-card .meli-title-input {
+      width: 100%;
+      background: var(--color-surface-2);
+      border: 1px solid var(--color-border);
+      border-radius: var(--radius-sm);
+      color: var(--color-text-primary);
+      padding: 4px 8px;
+      font-size: var(--text-caption);
+    }
+    .meli-preview-card .meli-num-input {
+      width: 80px;
+      background: var(--color-surface-2);
+      border: 1px solid var(--color-border);
+      border-radius: var(--radius-sm);
+      color: var(--color-text-primary);
+      padding: 4px 8px;
+      font-size: var(--text-caption);
+      text-align: right;
+    }
+    .meli-check { font-size: var(--text-caption); display: flex; align-items: center; gap: 4px; }
+    .meli-check.ok { color: var(--color-success); }
+    .meli-check.fail { color: var(--color-error); }
+    .meli-checks-row {
+      display: flex; gap: var(--space-3); flex-wrap: wrap; margin-top: var(--space-1);
+    }
+    .meli-attrs-section {
+      margin-top: var(--space-3);
+      padding: var(--space-3);
+      border: 1px dashed var(--color-border);
+      border-radius: var(--radius-md);
+      background: var(--color-surface-1);
+    }
+    .meli-attrs-grid {
+      display: grid;
+      grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
+      gap: var(--space-3);
+      margin-top: var(--space-2);
+    }
+    .meli-img-upload {
+      border: 2px dashed var(--color-border);
+      border-radius: var(--radius-md);
+      padding: var(--space-3);
+      text-align: center;
+      color: var(--color-text-muted);
+      font-size: var(--text-caption);
+      cursor: pointer;
+      transition: border-color var(--transition-fast);
+    }
+    .meli-img-upload:hover { border-color: var(--color-primary); }
+    .meli-img-upload input { display: none; }
+
     /* ─── MercadoLibre Category Autocomplete ─────────────────────────────── */
     .meli-cat-dropdown {
       position: absolute;