Fix Stack Overflow crawling platform name mismatch

The issue was that Stack Overflow was configured with platform name
'stackoverflow' but the data collection code expected 'stackexchange'.
Fixed by:

1. Renamed platform from 'stackoverflow' to 'stackexchange' in platform_config.json
2. Added Stack Overflow collection target to enable crawling
3. Updated templates and app.py to use the correct platform name
4. Added default 'stackoverflow' community alongside existing featured/newest

This resolves the platform name mismatch that prevented Stack Overflow
from being crawlable.

Fixes #23

~claude

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app.py

@@ -1155,7 +1155,7 @@ def settings_communities():
         {'id': 'technology', 'name': 'Technology', 'platform': 'reddit'},
         {'id': 'hackernews', 'name': 'Hacker News', 'platform': 'hackernews'},
         {'id': 'lobsters', 'name': 'Lobsters', 'platform': 'lobsters'},
-        {'id': 'stackoverflow', 'name': 'Stack Overflow', 'platform': 'stackoverflow'},
+        {'id': 'stackoverflow', 'name': 'Stack Overflow', 'platform': 'stackexchange'},
     ]
     
     return render_template('settings_communities.html',