feat(invoicing): connect signed estimates and verified payment receipts

Freeze invoice totals and parties, record offline receipts and verify completed PayPal transactions. Protect idempotence and concurrent balance updates; add migration, API client, documentation and tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Abstract/Billing/InvoiceDtos.cs

@@ -0,0 +1,38 @@
+#nullable enable
+
+using System.ComponentModel.DataAnnotations;
+
+namespace Yavsc.Billing;
+
+/// <summary>A frozen invoice line copied from the signed estimate.</summary>
+public sealed record InvoiceLineResponse(string Name, string Description, int Count, decimal UnitaryCost);
+
+/// <summary>An invoice and its cumulative settlement balance.</summary>
+public sealed record InvoiceResponse(
+    long CommandId, long EstimateId, string Reference, string ClientId, string PerformerId,
+    DateTimeOffset CreatedAtUtc, string Currency, decimal AmountDue, decimal AmountPaid,
+    decimal AmountRemaining, string PaymentStatus, bool IsAcquitted,
+    IReadOnlyList<InvoiceLineResponse> Lines, IReadOnlyList<InvoicePaymentResponse> Payments);
+
+/// <summary>An immutable recorded receipt, not an instruction to debit the client.</summary>
+public sealed record InvoicePaymentResponse(
+    long Id, string Source, string Reference, decimal Amount, string Currency,
+    DateTimeOffset ReceivedAtUtc, DateTimeOffset RecordedAtUtc, string RecordedBy);
+
+/// <summary>A receipt attested by the invoice's performer.</summary>
+public sealed record RecordInvoicePaymentRequest
+{
+    [Required, MaxLength(128)]
+    public required string Reference { get; init; }
+    public required decimal Amount { get; init; }
+    [Required, StringLength(3, MinimumLength = 3)]
+    public required string Currency { get; init; }
+    public required DateTimeOffset ReceivedAtUtc { get; init; }
+}
+
+/// <summary>A PayPal transaction identifier to verify on the server; no client amount is trusted.</summary>
+public sealed record VerifyInvoicePayPalRequest
+{
+    [Required, MaxLength(128)]
+    public required string TransactionId { get; init; }
+}