feat(06-01): add web channel type, Redis key, ORM models, migration, and tests

- Add ChannelType.WEB = 'web' to shared/models/message.py
- Add webchat_response_key() to shared/redis_keys.py
- Create WebConversation and WebConversationMessage ORM models (SQLAlchemy 2.0)
- Create migration 008_web_chat.py with RLS, indexes, and channel_type CHECK update
- Pop conversation_id/portal_user_id extras in handle_message before model_validate
- Add web case to _build_response_extras and _send_response (Redis pub-sub publish)
- Import webchat_response_key in orchestrator/tasks.py
- Write 19 unit tests covering CHAT-01 through CHAT-05 (all pass)

packages/shared/shared/models/message.py

@@ -26,6 +26,7 @@ class ChannelType(StrEnum):
     TEAMS = "teams"
     TELEGRAM = "telegram"
     SIGNAL = "signal"
+    WEB = "web"
 
 
 class MediaType(StrEnum):