fix(downloads): stop repeating an extension the name already carries

A voice comment's display name is the generated file name, extension
included, so appending the extension again downloaded it as
<uuid>.webm.webm. Both download paths, the single asset route and the
project zip, now append only when the name does not already end in it.

tests/api/lib-video-assets.test.ts

@@ -19,6 +19,7 @@ import {
   extractVideoKeyFromProxyUrl,
   getVideoAssetAccessContext,
   sanitizeAssetDisplayName,
+  withFileExtension,
   SAFE_BUNNY_VIDEO_ID,
 } from '@/lib/video-assets';
 import { apiRequest } from '../helpers/request';
@@ -83,6 +84,28 @@ describe('sanitizeAssetDisplayName', () => {
   });
 });
 
+// A voice comment is stored under its generated file name, so its display name
+// already ends in .webm and every download was named `<uuid>.webm.webm`.
+describe('withFileExtension', () => {
+  it('leaves a name that already ends in the extension alone', () => {
+    expect(withFileExtension('33661b60-b658-47f9-bd40-71c32a0a2fdb.webm', '.webm')).toBe(
+      '33661b60-b658-47f9-bd40-71c32a0a2fdb.webm'
+    );
+  });
+
+  it('ignores the case the extension was written in', () => {
+    expect(withFileExtension('Take 2.WEBM', '.webm')).toBe('Take 2.WEBM');
+  });
+
+  it('appends when the name carries no extension', () => {
+    expect(withFileExtension('Voice Comment', '.webm')).toBe('Voice Comment.webm');
+  });
+
+  it('appends when the name ends in a different extension', () => {
+    expect(withFileExtension('clip.mp4', '.webm')).toBe('clip.mp4.webm');
+  });
+});
+
 describe('proxy URL extraction', () => {
   it('derives the image key and file name from a canonical image URL', () => {
     expect(extractImageKeyFromProxyUrl(IMAGE_URL)).toBe(