Fix handle_completed — guard against invalid completion_on timestamp

qBittorrent returns completion_on = -1 for torrents that were never
completed. datetime.fromtimestamp(-1) throws OSError on Windows.
Added explicit check for negative values and try/except for safety.

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

Seedbox/70 Manager.py

@@ -110,11 +110,15 @@ def handle_completed(qbt, cursor):
     """
     removed = 0
     for t in qbt.torrents_info():
-        if not t.completion_on:
+        if not t.completion_on or t.completion_on < 0:
+            continue
+
+        try:
+            completed_dt = datetime.fromtimestamp(t.completion_on)
+        except (OSError, ValueError, OverflowError):
             continue
 
         thash = t.hash.lower()
-        completed_dt = datetime.fromtimestamp(t.completion_on)
 
         try:
             qbt.torrents_delete(torrent_hashes=thash, delete_files=False)