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>

templates/post_detail.html

@@ -105,7 +105,7 @@
                         🐙 View on GitHub
                     {% elif post.platform == 'devto' %}
                         📝 View on Dev.to
-                    {% elif post.platform == 'stackoverflow' %}
+                    {% elif post.platform == 'stackexchange' %}
                         📚 View on Stack Overflow
                     {% else %}
                         🔗 View Original Source