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/pastemoni.py

@@ -275,6 +275,8 @@ async def _poll_once(bot, key: str, announce: bool = True) -> None:
                 title = item.get("title") or "(untitled)"
                 snippet = item.get("snippet", "")
                 url = item.get("url", "")
+                if url:
+                    url = await bot.shorten_url(url)
                 parts = [f"[{tag}] {title}"]
                 if snippet:
                     parts.append(snippet)