feat: add URL shortening to subscription announcements

Bot.shorten_url() method delegates to flaskpaste plugin when loaded.
RSS, YouTube, and pastemoni announcements auto-shorten links.
Includes test_flaskpaste.py (9 cases) and FakeBot updates in 3 test files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/youtube.py

@@ -394,6 +394,8 @@ async def _poll_once(bot, key: str, announce: bool = True) -> None:
         for item in shown:
             title = _truncate(item["title"]) if item["title"] else "(no title)"
             link = item["link"]
+            if link:
+                link = await bot.shorten_url(link)
             # Build metadata suffix
             parts = []
             dur = durations.get(item["id"], 0)