Add CRM features: Additional Contacts, Lead Source, Ship-To Address; update Help docs

- New CustomerContact entity + migration (AddCustomerContactsAndCrmFields)
- Customer.LeadSource + ShipToAddress/City/State/ZipCode/Country fields
- Additional Contacts card on Customer Details with AJAX add/edit/delete
- Lead Source dropdown on Create/Edit; Ship-To section on Create/Edit
- Customer Details: side-by-side billing/ship-to when ship-to is set
- Help docs: Customers (contacts, ship-to, lead source, preferred powders, outstanding pickups)
- Help docs: Jobs (clone job, project name), Quotes (project name), Invoices (project name), Inventory (low stock clickable filter)
- HelpKnowledgeBase.cs updated for all features above

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Help/Inventory.cshtml

@@ -415,7 +415,11 @@
                     An alert banner is shown on the item's Details page prompting you to use Stock Adjustment to add inventory.
                 </li>
             </ul>
-            <p>Low Stock and Out of Stock items appear in the Inventory Alerts section on the Dashboard and in the Operations Report. Use the <strong>Low Stock</strong> filter on the Inventory list to see only items needing attention.</p>
+            <p>
+                Low Stock and Out of Stock items appear in the Inventory Alerts section on the Dashboard and in the Operations Report.
+                The <strong>Low Stock</strong> stat card at the top of the Inventory page is clickable &mdash; click it to instantly
+                filter the list to only items needing attention. Click it again (or clear the filter) to return to the full list.
+            </p>
             <div class="alert alert-permanent alert-warning d-flex gap-2 mb-0" role="alert">
                 <i class="bi bi-exclamation-triangle-fill flex-shrink-0 mt-1"></i>
                 <div>