Fix KeyError when active_user_count missing from API response

Reddit's API no longer consistently returns the `active_user_count`
field in subreddit data. Using dict bracket access (`d['active_user_count']`)
raises a KeyError when the field is absent. Switch to `d.get()` which
returns None for missing keys, allowing the existing fallback to -1.

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

Author: johntrandall

redditwarp/models/subreddit.py

@@ -212,7 +212,7 @@ def __init__(self, d: Mapping[str, Any]) -> None:
 
         self.subscriber_count: int = d['subscribers']
         ("")
-        self.viewing_count: int = -1 if (x := d['active_user_count']) is None else x
+        self.viewing_count: int = -1 if (x := d.get('active_user_count')) is None else x
         ("""
             The number of online users who are subscribed to the subreddit.