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.Web/Controllers/InvoicesController.cs

@@ -1851,6 +1851,54 @@ public class InvoicesController : Controller
         return $"{prefix}{(maxNum + 1):D4}";
     }
 
+    /// <summary>
+    /// Returns the customer's payment terms, derived due date, and early-payment discount info
+    /// for the Invoice Create form so JavaScript can auto-populate those fields on customer selection.
+    /// </summary>
+    [HttpGet]
+    public async Task<IActionResult> GetCustomerPaymentTerms(int customerId)
+    {
+        var customer = await _unitOfWork.Customers.GetByIdAsync(customerId);
+        if (customer == null) return NotFound();
+
+        var invoiceDate = DateTime.Today;
+        var dueDate     = PaymentTermsParser.CalculateDueDate(customer.PaymentTerms, invoiceDate);
+        var (discountPercent, discountDays) = PaymentTermsParser.ParseEarlyPaymentDiscount(customer.PaymentTerms);
+
+        return Json(new
+        {
+            paymentTerms               = customer.PaymentTerms,
+            dueDate                    = dueDate?.ToString("yyyy-MM-dd"),
+            earlyPaymentDiscountPercent = discountPercent,
+            earlyPaymentDiscountDays   = discountDays,
+            isTaxExempt                = customer.IsTaxExempt
+        });
+    }
+
+    /// <summary>
+    /// Returns the default active tax rate for the current company, or zero for tax-exempt customers.
+    /// Called by the Invoice Create form when the customer selection changes.
+    /// </summary>
+    [HttpGet]
+    public async Task<IActionResult> GetTaxRateForCustomer(int customerId)
+    {
+        var companyId = _tenantContext.GetCurrentCompanyId() ?? 0;
+        var customer  = await _unitOfWork.Customers.GetByIdAsync(customerId);
+        if (customer == null) return NotFound();
+
+        if (customer.IsTaxExempt)
+            return Json(new { taxPercent = 0m, taxRateName = (string?)null });
+
+        var defaultRate = await _unitOfWork.TaxRates
+            .FirstOrDefaultAsync(r => r.IsDefault && r.IsActive && !r.IsDeleted);
+
+        return Json(new
+        {
+            taxPercent  = defaultRate?.Rate ?? 0m,
+            taxRateName = defaultRate?.Name
+        });
+    }
+
     /// <summary>
     /// Populates ViewBag data used by both Create GET and Create POST (on validation failure re-display):
     /// — Active customer list for the customer dropdown.