Add custom source input for manually adding communities

- Add 'Custom Source' text input field to polling form
- Allows manual entry of subreddits (r/subreddit) or RSS URLs
- Custom input overrides dropdown selection if filled
- Dropdown becomes optional when custom source is entered
- Backend prioritizes custom_source_id over dropdown source_id

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

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

app.py

@@ -1347,9 +1347,14 @@ def admin_polling_add():
 
     platform = request.form.get('platform')
     source_id = request.form.get('source_id')
+    custom_source_id = request.form.get('custom_source_id')
     display_name = request.form.get('display_name')
     poll_interval = int(request.form.get('poll_interval', 60))
 
+    # Use custom source if provided, otherwise use dropdown
+    if custom_source_id and custom_source_id.strip():
+        source_id = custom_source_id.strip()
+
     if not platform or not source_id or not display_name:
         flash('Missing required fields', 'error')
         return redirect(url_for('admin_polling'))