Patch export/import for missing fields; add CustomerContacts export

- DataExportController + AccountDataExportController: add ProjectName to
  Jobs, Quotes, Invoices (XLSX + CSV); add LeadSource + ShipTo fields to
  Customers (XLSX + CSV); add CustomerContacts sheet/CSV (new)
- Both export views: add Customer Contacts checkbox (checked by default)
- CustomerImportDto: add LeadSource + ShipTo* fields
- JobImportDto: add ProjectName
- QuoteImportDto: add ProjectName
- InvoiceImportDto: add Project Name (dual-name alias for round-trip)
- CsvImportService: wire all new import fields to entity creation;
  also patch invoice update path for ProjectName
- Add scripts/purge_imported_data.sql (dry-run T-SQL for data cleanup)

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

src/PowderCoating.Application/DTOs/Import/InvoiceImportDto.cs

@@ -33,6 +33,9 @@ public class InvoiceImportDto
     [Name("DueDate")]
     public DateTime? DueDate { get; set; }
 
+    [Name("Project Name", "ProjectName")]
+    public string? ProjectName { get; set; }
+
     [Name("SubTotal")]
     public decimal SubTotal { get; set; }