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>

APP/src/app/features/payee1099/services/payee1099-api.service.ts

@@ -39,4 +39,19 @@ export class Payee1099ApiService {
   revealTin(id: number): Observable<{ tin: string | null }> {
     return this.http.get<{ tin: string | null }>(`${this.endpoint}/${id}/tin`);
   }
+
+  uploadW9(id: number, file: File): Observable<void> {
+    const form = new FormData();
+    form.append('file', file);
+    return this.http.post<void>(`${this.endpoint}/${id}/w9`, form);
+  }
+
+  /**
+   * Fetches the stored W-9 as a Blob via HttpClient so the auth interceptor attaches
+   * the JWT. A plain window.open on the API URL would be an unauthenticated browser
+   * navigation and the API's permission gate would reject it.
+   */
+  downloadW9(id: number): Observable<Blob> {
+    return this.http.get(`${this.endpoint}/${id}/w9`, { responseType: 'blob' });
+  }
 }