feat: append source domain fragment to alert short URLs

Short URLs now include the original source domain as a URL fragment,
e.g. https://paste.mymx.me/s/foo#github.com, so the destination is
visible before clicking.

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

plugins/alert.py

@@ -12,6 +12,7 @@ import urllib.request
 from datetime import datetime, timezone
 from html.parser import HTMLParser
 from pathlib import Path
+from urllib.parse import urlparse
 
 from derp.http import urlopen as _urlopen
 from derp.plugin import command, event
@@ -1909,7 +1910,8 @@ async def _poll_once(bot, key: str, announce: bool = True) -> None:
                             None, fp.shorten_url, bot, url,
                         )
                         if short_url != url:
-                            display_url = short_url
+                            domain = urlparse(url).hostname or ""
+                            display_url = f"{short_url}#{domain}" if domain else short_url
                         else:
                             short_url = ""
                     except Exception:
@@ -2104,14 +2106,17 @@ async def cmd_alert(bot, message):
             title = _truncate(title) if title else "(no title)"
             if extra:
                 title = f"{title} | {extra}"
-            display_url = short_url or url
+            domain = urlparse(url).hostname or "" if url else ""
+            display_url = (f"{short_url}#{domain}" if short_url and domain
+                           else short_url or url)
             if fp and url and not short_url:
                 try:
                     new_short = await loop.run_in_executor(
                         None, fp.shorten_url, bot, url,
                     )
                     if new_short != url:
-                        display_url = new_short
+                        display_url = (f"{new_short}#{domain}" if domain
+                                       else new_short)
                         db.execute(
                             "UPDATE results SET short_url = ? WHERE id = ?",
                             (new_short, row_id),