Add Reddit video support

Reddit links now download and post like the other platforms.

Two changes were needed, not just the URL pattern:

- Regex: yt-dlp's Reddit extractor only matches /comments/ permalinks, so
  the app's /r/<sub>/s/<code> share links and redd.it / v.redd.it
  shortlinks are listed explicitly — they redirect to the canonical URL,
  which yt-dlp follows. Without them the link is silently dropped.

- Format selector: Reddit publishes video and audio as separate DASH/HLS
  streams with no muxed rendition, so every `best...` term failed and
  yt-dlp aborted with "Requested format is not available". Added a
  bv*+ba merge tail (and a bare bv* for silent videos) to the normal and
  /force selectors. Muxed hosts still resolve at the earlier terms, so
  YouTube/IG/TikTok/X selection is unchanged.

Posts with no video report "No media found", already in the silent list,
so ordinary Reddit links shared in chat draw no reply.

README.md

@@ -1,8 +1,8 @@
 # signal-bot
 
 A Signal group-chat bot that grabs videos from links people post (X/Twitter,
-Instagram, YouTube, TikTok) and posts them back into the chat, plus a couple of
-toys for messing with the last video.
+Instagram, YouTube, TikTok, Reddit) and posts them back into the chat, plus a
+couple of toys for messing with the last video.
 
 ## What it does
 
@@ -16,7 +16,9 @@ toys for messing with the last video.
   to refresh Instagram auth without touching the server.
 
 Supported links: `x.com` / `twitter.com` (and `fxtwitter`/`vxtwitter`/`fixupx`
-wrappers), `instagram.com/reel|p`, `youtube.com` / `youtu.be` / Shorts, and TikTok.
+wrappers), `instagram.com/reel|p`, `youtube.com` / `youtu.be` / Shorts, TikTok,
+and Reddit (`/comments/` permalinks, `/s/` app share links, `redd.it` and
+`v.redd.it` shortlinks). Reddit posts with no video stay silent.
 
 ## How it's wired