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/settings_communities.html

@@ -79,7 +79,7 @@
     .platform-icon.reddit { background: #ff4500; }
     .platform-icon.hackernews { background: #ff6600; }
     .platform-icon.lobsters { background: #ac130d; }
-    .platform-icon.stackoverflow { background: #f48024; }
+    .platform-icon.stackexchange { background: #f48024; }
 
     .community-grid {
         display: grid;
@@ -269,7 +269,7 @@
                 <div class="platform-group">
                     <h3>
                         <span class="platform-icon {{ platform }}">
-                            {% if platform == 'reddit' %}R{% elif platform == 'hackernews' %}H{% elif platform == 'lobsters' %}L{% elif platform == 'stackoverflow' %}S{% endif %}
+                            {% if platform == 'reddit' %}R{% elif platform == 'hackernews' %}H{% elif platform == 'lobsters' %}L{% elif platform == 'stackexchange' %}S{% endif %}
                         </span>
                         {{ platform|title }}
                     </h3>