feat(03-05): mount Phase 3 API routers on gateway FastAPI app

- Import all 6 Phase 3 routers from shared.api (portal, billing, channels, llm_keys, usage, webhook)
- Add include_router() calls after existing whatsapp_router
- Update module docstring to document portal API endpoints

packages/gateway/gateway/main.py

@@ -3,6 +3,7 @@ Channel Gateway — FastAPI application.
 
 Mounts the slack-bolt AsyncApp as a sub-application at /slack/events.
 Registers the WhatsApp webhook router at /whatsapp/webhook.
+Serves all Phase 3 portal API routes under /api/portal/* and /api/webhooks/*.
 
 Port: 8001
 
@@ -10,6 +11,12 @@ Endpoints:
   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  /api/portal/*                      — Portal management API (tenants, agents, billing, etc.)
+  GET  /api/portal/billing/*              — Stripe billing endpoints
+  GET  /api/portal/channels/*             — Channel connection endpoints (Slack OAuth, WhatsApp)
+  GET  /api/portal/tenants/{id}/llm-keys  — BYO LLM key management
+  GET  /api/portal/usage/*                — Usage and cost analytics
+  POST /api/webhooks/*                    — Stripe webhook receiver
   GET  /health                            — Health check
 
 Startup sequence:
@@ -18,7 +25,8 @@ Startup sequence:
   3. Register Slack event handlers
   4. Mount slack-bolt request handler at /slack/events
   5. Include WhatsApp router
-  6. Expose /health
+  6. Include Phase 3 portal API routers
+  7. Expose /health
 """
 
 from __future__ import annotations
@@ -32,6 +40,14 @@ from slack_bolt.async_app import AsyncApp
 
 from gateway.channels.slack import register_slack_handlers
 from gateway.channels.whatsapp import whatsapp_router
+from shared.api import (
+    billing_router,
+    channels_router,
+    llm_keys_router,
+    portal_router,
+    usage_router,
+    webhook_router,
+)
 from shared.config import settings
 from shared.db import async_session_factory
 
@@ -88,6 +104,16 @@ slack_handler = AsyncSlackRequestHandler(slack_app)
 # ---------------------------------------------------------------------------
 app.include_router(whatsapp_router)
 
+# ---------------------------------------------------------------------------
+# Register Phase 3 portal API routers
+# ---------------------------------------------------------------------------
+app.include_router(portal_router)
+app.include_router(billing_router)
+app.include_router(channels_router)
+app.include_router(llm_keys_router)
+app.include_router(usage_router)
+app.include_router(webhook_router)
+
 
 # ---------------------------------------------------------------------------
 # Routes