Fix bug pass: async offload, resource leaks, media/parse correctness, deploy hardening

bot.py:
- Run all yt-dlp/ffmpeg/ffprobe/file-IO/base64 work off the event loop via
  asyncio.to_thread, bounded by a Semaphore(2); the loop no longer freezes
  bot-wide during downloads/encodes.
- download_attachments=False; VideoTracker fetches only video attachments lazily
  instead of the library base64-ing every attachment of every message.
- last_video keyed per (group, sender) with group-latest fallback so /speed and
  /rev stop silently targeting a stranger's video; proactive TTL sweeps free the
  big base64 blobs and bound recent_urls growth.
- _reencode guards 0/NaN ffprobe duration and adds -maxrate/-bufsize; /rev now
  size-checks + re-encodes + faststart like the other paths.
- Twitter URL regex no longer merges space-separated links (keeps i/web/status);
  dedupe repeated URLs within a message; mark-handled only on success/no-media so
  a corrective edit can retry; 'unsupported url' surfaced instead of silently dropped.
- All sends wrapped (catch SendMessageError); base64 decode guarded; edit/sync
  attachment envelopes handled; /cookies temp file created 0600.

deploy:
- Pin signalbot==1.1.0, yt-dlp floor, add missing yt-dlp-ejs.
- Pin signal-cli-rest-api by digest + add /v1/health healthcheck.
- Restart=always (the library never exits non-zero when wedged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

signal-bot.service.example

@@ -13,7 +13,9 @@ Environment=SIGNAL_SERVICE=127.0.0.1:8080
 # Optional: comma-separated numbers allowed to run /cookies (admin command).
 # Environment=BOT_ADMINS=+15551234567
 ExecStart=/home/YOUR_USER/signal-bot/venv/bin/python bot.py
-Restart=on-failure
+# always (not on-failure): the bot's library swallows exceptions in its own run
+# loop and never exits non-zero even when wedged, so on-failure would never bounce it.
+Restart=always
 RestartSec=10
 
 [Install]