feat(atlas): import catalog, customers and historical sales + viewer

- Add scripts/import_atlas_data.py to load Atlas data from Excel files
- Import 6,206 inventory items, 251 customers and 4,582 historical sales
- Create historical_sales table in tenant DB
- Add /pos/historical-sales page and /pos/api/historical-sales endpoint
- Link in reports sidebar for easy access

pos/templates/reports.html

@@ -130,6 +130,16 @@
           <span>Reportes</span>
         </a>
 
+        <a href="/pos/historical-sales" class="nav-item">
+          <svg class="nav-item__icon" viewBox="0 0 18 18" fill="none" stroke="currentColor" stroke-width="1.5">
+            <rect x="1" y="4" width="16" height="12" rx="1"/>
+            <line x1="1" y1="8" x2="17" y2="8"/>
+            <line x1="5" y1="2" x2="5" y2="4"/>
+            <line x1="13" y1="2" x2="13" y2="4"/>
+          </svg>
+          <span>Ventas Históricas</span>
+        </a>
+
         <div class="nav-section-label">Sistema</div>
 
         <a href="/pos/config" class="nav-item">