feat(1099): wire W-9 document upload/view for recipients

Adds POST/GET payee-1099/{id}/w9, mirroring the expense-receipt upload:
IFileStorage saves to finance/w9/{id}{ext}, content-type derived from the
blob extension. Frontend dialog (edit mode) gains a W-9 file input and an
auth-correct blob "View W-9" link. Payee1099Service ctor now takes
IFileStorage; tests updated with an in-memory FakeStorage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API/Services/IPayee1099Service.cs

@@ -10,4 +10,8 @@ public interface IPayee1099Service
     Task DeleteAsync(int id);
     /// <summary>Full decrypted TIN. Caller must be authorized (gated at controller).</summary>
     Task<string?> RevealTinAsync(int id);
+    /// <summary>Stores the uploaded W-9 blob and records its path. Throws KeyNotFoundException if the payee is missing.</summary>
+    Task SaveW9Async(int id, Stream content, string fileName);
+    /// <summary>Opens the stored W-9 blob; null when none is attached.</summary>
+    Task<(Stream stream, string contentType)?> OpenW9Async(int id);
 }