Set poll sources disabled by default with better interval options

- Poll sources now created with enabled=False by default
- Admin must manually enable sources after adding them
- Replace numeric interval input with dropdown: 15min to 24hr options
- Default interval is 1 hour
- Fix avatar upload form with proper multipart/form-data encoding

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

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

app.py

@@ -1360,13 +1360,13 @@ def admin_polling_add():
         flash(f'Source {platform}:{source_id} already exists', 'warning')
         return redirect(url_for('admin_polling'))
 
-    # Create new source
+    # Create new source (disabled by default)
     source = PollSource(
         platform=platform,
         source_id=source_id,
         display_name=display_name,
         poll_interval_minutes=poll_interval,
-        enabled=True,
+        enabled=False,
         created_by=current_user.id
     )