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.Core/Entities/Customer.cs

@@ -41,6 +41,17 @@ public class Customer : BaseEntity
     public bool IsActive { get; set; } = true;
     public DateTime? LastContactDate { get; set; }
 
+    // CRM fields
+    /// <summary>How the customer found the shop (Walk-In, Google Search, Customer Referral, etc.).</summary>
+    public string? LeadSource { get; set; }
+
+    // Ship-to / alternate address (separate from billing address above)
+    public string? ShipToAddress { get; set; }
+    public string? ShipToCity { get; set; }
+    public string? ShipToState { get; set; }
+    public string? ShipToZipCode { get; set; }
+    public string? ShipToCountry { get; set; }
+
     // Notification preferences
     public bool NotifyByEmail { get; set; } = true;
     // NotifyBySms is only set to true after explicit staff-recorded consent (TCPA compliance)
@@ -55,4 +66,5 @@ public class Customer : BaseEntity
 
     public virtual ICollection<NotificationLog> NotificationLogs { get; set; } = new List<NotificationLog>();
     public virtual ICollection<Invoice> Invoices { get; set; } = new List<Invoice>();
+    public virtual ICollection<CustomerContact> CustomerContacts { get; set; } = new List<CustomerContact>();
 }