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>

tests/test_rss.py

@@ -166,6 +166,9 @@ class _FakeBot:
     async def reply(self, message, text: str) -> None:
         self.replied.append(text)
 
+    async def shorten_url(self, url: str) -> str:
+        return url
+
     def _is_admin(self, message) -> bool:
         return self._admin