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/TaxRatesController.cs

@@ -0,0 +1,139 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using PowderCoating.Core.Entities;
+using PowderCoating.Core.Interfaces;
+using PowderCoating.Shared.Constants;
+
+namespace PowderCoating.Web.Controllers;
+
+/// <summary>
+/// Manages named tax rates used to pre-fill the tax percent field on invoices when a taxable
+/// customer is selected. Only one rate may be marked as default at a time; that default is
+/// auto-applied via the GetTaxRateForCustomer AJAX endpoint on the Invoice Create form.
+/// </summary>
+[Authorize(Policy = AppConstants.Policies.CompanyAdminOnly)]
+public class TaxRatesController : Controller
+{
+    private readonly IUnitOfWork _unitOfWork;
+    private readonly ITenantContext _tenantContext;
+    private readonly ILogger<TaxRatesController> _logger;
+
+    public TaxRatesController(
+        IUnitOfWork unitOfWork,
+        ITenantContext tenantContext,
+        ILogger<TaxRatesController> logger)
+    {
+        _unitOfWork = unitOfWork;
+        _tenantContext = tenantContext;
+        _logger = logger;
+    }
+
+    /// <summary>Lists all tax rates for the current company.</summary>
+    public async Task<IActionResult> Index()
+    {
+        var rates = await _unitOfWork.TaxRates.GetAllAsync();
+        return View(rates.OrderBy(r => r.Name).ToList());
+    }
+
+    [HttpGet]
+    public IActionResult Create() => View(new TaxRate());
+
+    /// <summary>Creates a new tax rate. Enforces that only one rate is the default.</summary>
+    [HttpPost]
+    [ValidateAntiForgeryToken]
+    public async Task<IActionResult> Create(TaxRate model)
+    {
+        if (!ModelState.IsValid) return View(model);
+
+        if (model.IsDefault)
+            await ClearOtherDefaultsAsync(0);
+
+        await _unitOfWork.TaxRates.AddAsync(model);
+        await _unitOfWork.CompleteAsync();
+
+        TempData["Success"] = $"Tax rate \"{model.Name}\" created.";
+        return RedirectToAction(nameof(Index));
+    }
+
+    [HttpGet]
+    public async Task<IActionResult> Edit(int id)
+    {
+        var rate = await _unitOfWork.TaxRates.GetByIdAsync(id);
+        if (rate == null) return NotFound();
+        return View(rate);
+    }
+
+    /// <summary>Updates an existing tax rate. Clears default flag on other rates when IsDefault is set.</summary>
+    [HttpPost]
+    [ValidateAntiForgeryToken]
+    public async Task<IActionResult> Edit(int id, TaxRate model)
+    {
+        if (id != model.Id) return BadRequest();
+        if (!ModelState.IsValid) return View(model);
+
+        var rate = await _unitOfWork.TaxRates.GetByIdAsync(id);
+        if (rate == null) return NotFound();
+
+        if (model.IsDefault && !rate.IsDefault)
+            await ClearOtherDefaultsAsync(id);
+
+        rate.Name        = model.Name;
+        rate.Rate        = model.Rate;
+        rate.State       = model.State;
+        rate.Description = model.Description;
+        rate.IsDefault   = model.IsDefault;
+        rate.IsActive    = model.IsActive;
+
+        await _unitOfWork.CompleteAsync();
+
+        TempData["Success"] = $"Tax rate \"{rate.Name}\" updated.";
+        return RedirectToAction(nameof(Index));
+    }
+
+    /// <summary>Toggles IsActive without a full page reload.</summary>
+    [HttpPost]
+    [ValidateAntiForgeryToken]
+    public async Task<IActionResult> ToggleActive(int id)
+    {
+        var rate = await _unitOfWork.TaxRates.GetByIdAsync(id);
+        if (rate == null) return NotFound();
+
+        rate.IsActive = !rate.IsActive;
+        if (!rate.IsActive) rate.IsDefault = false;
+
+        await _unitOfWork.CompleteAsync();
+        return RedirectToAction(nameof(Index));
+    }
+
+    /// <summary>Soft-deletes a tax rate. Blocked when the rate is currently the default.</summary>
+    [HttpPost]
+    [ValidateAntiForgeryToken]
+    public async Task<IActionResult> Delete(int id)
+    {
+        var rate = await _unitOfWork.TaxRates.GetByIdAsync(id);
+        if (rate == null) return NotFound();
+
+        if (rate.IsDefault)
+        {
+            TempData["Error"] = "Cannot delete the default tax rate. Set another rate as default first.";
+            return RedirectToAction(nameof(Index));
+        }
+
+        await _unitOfWork.TaxRates.SoftDeleteAsync(id);
+        await _unitOfWork.CompleteAsync();
+
+        TempData["Success"] = $"Tax rate \"{rate.Name}\" deleted.";
+        return RedirectToAction(nameof(Index));
+    }
+
+    /// <summary>
+    /// Clears IsDefault on all rates except the one with <paramref name="exceptId"/>.
+    /// Called before saving a newly-designated default to enforce the single-default invariant.
+    /// </summary>
+    private async Task ClearOtherDefaultsAsync(int exceptId)
+    {
+        var others = await _unitOfWork.TaxRates.FindAsync(r => r.IsDefault && r.Id != exceptId);
+        foreach (var r in others)
+            r.IsDefault = false;
+    }
+}