Signup-to-release recording on the landing page and Quickstart !465

• docs/demo/: vhs tapes (signup, verify, ship) recorded as zerocool on gitbay.org; env.sh isolates the demo home and key; render.sh encodes and joins the frames, since vhs 0.12's ffmpeg step writes nothing under ffmpeg 9.
• Landing: the recording autoplays muted with controls (webm, mp4 fallback) and replaces the merge request picture, whose PNGs are removed. The quickstart block loses its # every command comment.
• /static/img/* serves gif, webm and mp4 by extension through ServeContent, which answers Range (Safari needs it for video). TestLandingImagesAreServed checks the type and a 206 on video; TestLandingRoutes expects the video.
• Quickstart wiki shows https://gitbay.org/static/img/demo.gif.
• Footer iOS link goes to the App Store.

Closes #253