feat: multi-team orchestrator - topology UI, team management, flag randomizer, public submit + leaderboard

- panel/teams.py: create/start/stop teams with isolated ports+creds, per-team receivers with CHALLENGE_PORT/CONTAINER env, flag randomization, submit validation + leaderboard
- panel/main.py: /api/teams, /api/teams/{idx}/randomize, /api/flag/submit, /api/leaderboard, /api/public/teams, /submit page
- panel/static/submit.html: public flag submission UI for teams
- receiver: _ch_port/_ch_container read .env per team, container name overrides
- .gitignore: exclude teams/, .venv, __pycache__

receiver/challenges/CDN.py

@@ -1,3 +1,4 @@
+import os
 import io
 import re
 import random
@@ -26,7 +27,7 @@ class CDN(Challenge):
     flag_location = 'flags/cdn.txt'     # host copy (written by your orchestrator)
     history_location = 'history/cdn.txt'
     container_flag_path = '/flag.txt'
-    container_name = 'cdn_container'            # adjust if your container name differs
+    container_name = os.environ.get('CHALLENGE_CONTAINER_CDN', 'cdn_container')            # adjust if your container name differs
 
     # Canonical ExifTool markers (NO app fallbacks allowed)
     _exif_must_have = (