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>

src/derp/bot.py

@@ -510,6 +510,17 @@ class Bot:
         """Send a CTCP ACTION (/me) to a target."""
         await self.send(target, f"\x01ACTION {text}\x01")
 
+    async def shorten_url(self, url: str) -> str:
+        """Shorten a URL via FlaskPaste. Returns original on failure."""
+        fp = self.registry._modules.get("flaskpaste")
+        if not fp:
+            return url
+        loop = asyncio.get_running_loop()
+        try:
+            return await loop.run_in_executor(None, fp.shorten_url, self, url)
+        except Exception:
+            return url
+
     async def join(self, channel: str) -> None:
         """Join an IRC channel."""
         await self.conn.send(format_msg("JOIN", channel))