feat: microservices scaffold + Auth Service (Step 1)

- Add shared/ module: config, db, models, schemas, redis utilities
- Add Auth Service (services/auth/): register, login, refresh, me,
  ForwardAuth /verify endpoint for Traefik
- Add Traefik config: ACME/Cloudflare DNS-01, dynamic routing,
  ForwardAuth middleware, sticky sessions for WS Gateway
- Add service scaffolds: ws-gateway, chat, batch-agent, billing (READMEs)
- Add redis>=5.0.0 to requirements.txt
- Monolith app/ is untouched — strangler fig migration

shared/__init__.py

@@ -0,0 +1,5 @@
+"""Shared module — imported by all microservices.
+
+Contains DB engine/session, ORM models, Pydantic schemas, config,
+and Redis utilities. Changes here affect ALL services.
+"""