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>

platform_config.json

@@ -143,13 +143,20 @@
         }
       ]
     },
-    "stackoverflow": {
+    "stackexchange": {
       "name": "Stack Overflow",
       "icon": "📚",
       "color": "#f48024",
       "prefix": "",
       "supports_communities": false,
       "communities": [
+        {
+          "id": "stackoverflow",
+          "name": "Stack Overflow",
+          "display_name": "Stack Overflow",
+          "icon": "📚",
+          "description": "Programming Q&A community"
+        },
         {
           "id": "featured",
           "name": "Featured",
@@ -257,6 +264,12 @@
       "community": "https://hnrss.org/frontpage",
       "max_posts": 50,
       "priority": "low"
+    },
+    {
+      "platform": "stackexchange",
+      "community": "stackoverflow",
+      "max_posts": 50,
+      "priority": "medium"
     }
   ]
 }