Add video picker and Discord video attachments.

The picker accepts one MP4 or WebM, the thread lightbox zooms photos, and Discord gets the video as a file on a follow-up message so photo embeds still render.

internal/web/post_images_test.go

@@ -295,6 +295,19 @@ func TestPostVideoMultipartLifecycle(t *testing.T) {
 		t.Fatalf("streamed video upload = %+v", streamed)
 	}
 
+	rec = httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/questions/"+root.ID, nil)
+	for _, cookie := range cookies {
+		req.AddCookie(cookie)
+	}
+	handler.ServeHTTP(rec, req)
+	page := rec.Body.String()
+	if rec.Code != http.StatusOK ||
+		!strings.Contains(page, `<video src="`+root.Images[1].PublicURL) ||
+		!strings.Contains(page, `data-existing-video`) {
+		t.Fatalf("question page missing video player: %d %s", rec.Code, page)
+	}
+
 	rec = multipartPost(t, handler, "/posts/"+root.ID+"/edit", map[string][]string{
 		"_csrf":                      {csrf},
 		"body":                       {"Keep the clip."},