Discord channel: split oversized digests across embeds (#220)

A mass-backup webhook that exceeded ~2 KB used to be silently
truncated by `desc = message[:MAX_EMBED_DESC]` with MAX_EMBED_DESC
set to 2048 — half of Discord's real description limit and far
below what a multi-VM backup digest produces. The trailing jobs
just vanished from the channel.

Bring the channel up to Discord's actual webhook contract:

* description limit raised to the real 4096-char cap
* if the body still doesn't fit, split it on line boundaries into
  one embed per chunk so every backup entry is preserved
* keep title + fields on the first embed only; attach the footer
  and timestamp to the last embed so the rendered card has the
  normal head/tail framing even when split across many embeds
* enforce Discord's 6000-char-per-embed cap (title + description +
  every field name+value) — only kicks in when many large fields
  combine with a chunk already near the description ceiling
* batch up to 10 embeds per webhook POST (Discord's per-message
  limit) and POST additional messages sequentially with a 0.4 s
  gap so a >10-embed digest doesn't trip the 5/2 s webhook rate
  limit

Verified with synthetic mass-backup payloads:
* 14 KB / 200 jobs → 4 embeds, 1 POST
* 60 KB / 60 lines → 15 embeds, 2 POSTs (10 + 5)

New AppImage SHA-256:
  16ad59ea63a64e5be460cd73f87315e8b39b756bf1c61f3cb2019e9fa3e76361

Closes #220.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AppImage/ProxMenux-Monitor.AppImage.sha256

@@ -1 +1 @@
-3b44eb1172b4b1b7e6a36d1c9f1cd5a237ec04d52543bb791358525b0653a402
+16ad59ea63a64e5be460cd73f87315e8b39b756bf1c61f3cb2019e9fa3e76361