Phase F: Customer/Vendor Statements, Payment Terms Parser, Tax Rates

F1: GetCustomerStatementAsync/GetVendorStatementAsync on IFinancialReportService;
    StatementLineDto; CustomerStatementDto/VendorStatementDto; Statement action on
    CustomersController + VendorsController; Statement views + PDF download via
    StatementPdfHelper (QuestPDF); Statement button on Customer/Vendor Details pages.

F2: PaymentTermsParser static helper (CalculateDueDate, ParseEarlyPaymentDiscount);
    EarlyPaymentDiscountPercent/Days on Invoice entity; GetCustomerPaymentTerms AJAX
    endpoint on InvoicesController auto-populates Terms + due date on customer select;
    early payment discount notice on Invoice Create.

F3: TaxRate entity (Name/Rate/State/IsDefault/IsActive, tenant-filtered);
    IUnitOfWork.TaxRates + UnitOfWork + ApplicationDbContext; TaxRatesController
    (Index/Create/Edit/Delete/ToggleActive, CompanyAdminOnly); GetTaxRateForCustomer
    AJAX endpoint; Tax Rates in Settings gear menu.

Also fixes AddVendorCredits migration: VendorCreditApplications FKs changed from
CASCADE to NoAction to resolve SQL Server error 1785 (multiple cascade paths).
Migration: AddPaymentTermsAndTaxRates applied locally; 200/200 unit tests pass.

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

src/PowderCoating.Infrastructure/Migrations/20260510035830_AddVendorCredits.cs

@@ -78,13 +78,13 @@ namespace PowderCoating.Infrastructure.Migrations
                         column: x => x.BillId,
                         principalTable: "Bills",
                         principalColumn: "Id",
-                        onDelete: ReferentialAction.Cascade);
+                        onDelete: ReferentialAction.NoAction);
                     table.ForeignKey(
                         name: "FK_VendorCreditApplications_VendorCredits_VendorCreditId",
                         column: x => x.VendorCreditId,
                         principalTable: "VendorCredits",
                         principalColumn: "Id",
-                        onDelete: ReferentialAction.Cascade);
+                        onDelete: ReferentialAction.NoAction);
                 });
 
             migrationBuilder.CreateTable(