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/app.py

@@ -201,6 +201,10 @@ def create_app():
     def pos_marketplace_external_callback():
         return render_template('marketplace_external.html')
 
+    @app.route('/pos/historical-sales')
+    def pos_historical_sales():
+        return render_template('historical_sales.html')
+
     @app.route('/pos/static/<path:filename>')
     def pos_static(filename):
         return send_from_directory('static', filename)