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/pages/payee-1099-page/payee-1099-page.component.html

@@ -146,6 +146,17 @@
         <kendo-datepicker [(value)]="form.w9ReceivedDate"></kendo-datepicker>
       </label>
 
+      <!-- W-9 document upload/view: edit mode only (a new record is saved first, then re-opened to attach). -->
+      <div *ngIf="editingId != null" class="flex flex-col gap-1 md:col-span-2">
+        <span>W-9 Document / W-9 文件</span>
+        <input type="file" accept="image/jpeg,image/png,image/webp,application/pdf"
+          *appHasPermission="{ module: 'Form1099', action: 'write' }"
+          (change)="onW9FileSelected($event)" />
+        <span class="hint-text-sm">Upload W-9 / 上傳 W-9</span>
+        <button *ngIf="editingHasW9" kendoButton type="button" fillMode="link" class="self-start"
+          (click)="viewW9()">View W-9 / 檢視 W-9</button>
+      </div>
+
       <label class="flex flex-col gap-1 md:col-span-2">
         Notes / 備註
         <kendo-textarea [(ngModel)]="form.notes" [rows]="3"></kendo-textarea>