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.Web/Views/DataExport/Index.cshtml

@@ -180,6 +180,13 @@
                                     <i class="bi bi-people me-1 text-muted"></i>Customers
                                 </label>
                             </div>
+                            <div class="form-check mb-1">
+                                <input class="form-check-input sheet-check" type="checkbox" name="sheets"
+                                       value="CustomerContacts" id="chkCustomerContacts" checked />
+                                <label class="form-check-label" for="chkCustomerContacts">
+                                    <i class="bi bi-person-lines-fill me-1 text-muted"></i>Customer Contacts
+                                </label>
+                            </div>
                             <div class="form-check mb-1">
                                 <input class="form-check-input sheet-check" type="checkbox" name="sheets"
                                        value="Jobs" id="chkJobs" checked />