feat(02-03): WhatsApp adapter with business-function scoping and router registration

- Register whatsapp_router in gateway main.py (GET + POST /whatsapp/webhook)
- Implement is_clearly_off_topic() tier 1 keyword scoping gate
- Implement build_off_topic_reply() canned redirect message builder
- Full webhook handler: verify -> normalize -> tenant -> rate limit -> dedup -> scope -> media -> dispatch
- Outbound delivery via send_whatsapp_message() and send_whatsapp_media()
- Media download from Meta API and storage in MinIO with tenant-prefixed keys
- 14 new passing scoping tests

packages/gateway/gateway/main.py

@@ -2,20 +2,23 @@
 Channel Gateway — FastAPI application.
 
 Mounts the slack-bolt AsyncApp as a sub-application at /slack/events.
-All other channels will be added as additional sub-applications in Phase 2.
+Registers the WhatsApp webhook router at /whatsapp/webhook.
 
 Port: 8001
 
 Endpoints:
-  POST /slack/events  — Slack Events API webhook (handled by slack-bolt)
-  GET  /health        — Health check
+  POST /slack/events          — Slack Events API webhook (handled by slack-bolt)
+  GET  /whatsapp/webhook      — WhatsApp hub challenge verification
+  POST /whatsapp/webhook      — WhatsApp inbound message webhook
+  GET  /health                — Health check
 
 Startup sequence:
   1. Create Redis connection
   2. Create slack-bolt AsyncApp (signing_secret=...)
   3. Register Slack event handlers
   4. Mount slack-bolt request handler at /slack/events
-  5. Expose /health
+  5. Include WhatsApp router
+  6. Expose /health
 """
 
 from __future__ import annotations
@@ -28,6 +31,7 @@ from slack_bolt.adapter.fastapi.async_handler import AsyncSlackRequestHandler
 from slack_bolt.async_app import AsyncApp
 
 from gateway.channels.slack import register_slack_handlers
+from gateway.channels.whatsapp import whatsapp_router
 from shared.config import settings
 from shared.db import async_session_factory
 
@@ -79,6 +83,11 @@ register_slack_handlers(
 # ---------------------------------------------------------------------------
 slack_handler = AsyncSlackRequestHandler(slack_app)
 
+# ---------------------------------------------------------------------------
+# Register channel routers
+# ---------------------------------------------------------------------------
+app.include_router(whatsapp_router)
+
 
 # ---------------------------------------------------------------------------
 # Routes