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.Infrastructure/Data/ApplicationDbContext.cs

@@ -230,6 +230,8 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
     public DbSet<JobNote> JobNotes { get; set; }
     /// <summary>Free-text notes added to a customer record by staff; tenant-filtered with soft delete.</summary>
     public DbSet<CustomerNote> CustomerNotes { get; set; }
+    /// <summary>Additional contacts (billing, ops, drop-off) associated with a customer; tenant-filtered with soft delete.</summary>
+    public DbSet<CustomerContact> CustomerContacts { get; set; }
     /// <summary>Inventory items marked as frequently used for a customer; shown on Customer Details for faster quoting.</summary>
     public DbSet<CustomerPreferredPowder> CustomerPreferredPowders { get; set; }
     /// <summary>Audit trail of every status transition on a job, referencing the lookup-table statuses.</summary>