feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/static/css/inventory.css

@@ -732,6 +732,20 @@
       font-size: var(--text-caption);
     }
 
+    .btn--meli {
+      background: #FFE600;
+      color: #2D3277;
+      border-color: transparent;
+      font-weight: 700;
+    }
+    .btn--meli:hover {
+      background: #e6cf00;
+      color: #1a1f5c;
+    }
+    .btn--meli svg {
+      stroke: currentColor;
+    }
+
     /* =========================================================================
        DATA TABLE
        ========================================================================= */
@@ -1261,7 +1275,7 @@
     .inv-field label {
       font-size: var(--text-caption);
       font-weight: var(--font-weight-semibold);
-      color: var(--color-text-muted);
+      color: var(--color-text-secondary);
       letter-spacing: var(--tracking-wide);
       text-transform: uppercase;
     }
@@ -1282,6 +1296,23 @@
       box-shadow: 0 0 0 2px var(--color-primary-muted);
     }
 
+    .inv-field select {
+      padding: var(--space-2) var(--space-3);
+      background: var(--color-surface-1);
+      border: 1px solid var(--color-border);
+      border-radius: var(--radius-md);
+      color: var(--color-text-primary);
+      font-family: var(--font-body);
+      font-size: var(--text-body-sm);
+      width: 100%;
+    }
+
+    .inv-field select:focus {
+      outline: none;
+      border-color: var(--color-primary);
+      box-shadow: 0 0 0 2px var(--color-primary-muted);
+    }
+
     .count-row {
       display: flex;
       gap: var(--space-2);
@@ -1301,3 +1332,48 @@
 
     /* History table inside modal */
     .inv-modal .data-table { width: 100%; }
+
+    /* ─── MercadoLibre Category Autocomplete ─────────────────────────────── */
+    .meli-cat-dropdown {
+      position: absolute;
+      top: 100%;
+      left: 0;
+      right: 0;
+      z-index: 200;
+      background: var(--color-surface-1);
+      border: 1px solid var(--color-border);
+      border-radius: var(--radius-md);
+      box-shadow: var(--shadow-lg);
+      max-height: 240px;
+      overflow-y: auto;
+      margin-top: 4px;
+    }
+    .meli-cat-item {
+      padding: 10px 14px;
+      cursor: pointer;
+      font-size: var(--text-body-sm);
+      color: var(--color-text-primary);
+      border-bottom: 1px solid var(--color-border);
+      transition: background var(--transition-fast);
+      display: flex;
+      justify-content: space-between;
+      align-items: center;
+    }
+    .meli-cat-item:last-child { border-bottom: none; }
+    .meli-cat-item:hover,
+    .meli-cat-item.is-active {
+      background: var(--color-surface-2);
+    }
+    .meli-cat-item .cat-id {
+      font-size: var(--text-caption);
+      color: var(--color-text-muted);
+      margin-left: 8px;
+      font-family: var(--font-mono);
+    }
+    .meli-cat-loading,
+    .meli-cat-empty {
+      padding: 12px 14px;
+      font-size: var(--text-caption);
+      color: var(--color-text-muted);
+      text-align: center;
+    }