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/Invoices.cshtml

@@ -59,6 +59,7 @@
                 <li class="mb-2">Select the customer and then select the job this invoice is for.</li>
                 <li class="mb-2">Add or adjust line items as needed.</li>
                 <li class="mb-2">Set the invoice date, due date, and any notes.</li>
+                <li class="mb-2">Optionally enter a <strong>Project Name</strong>. When creating from a job, this pre-fills from the job's project name automatically.</li>
                 <li class="mb-2">Click <strong>Save Invoice</strong>.</li>
             </ol>