feat(billing): extract Billing Service (Step 4)

- stripe_service: checkout sessions, webhook handling, subscription CRUD
- tier_manager: feature matrix (4 tiers), quota enforcement, rate limits
- routes: checkout, webhook (no auth), subscription, tier query, features
- Traefik header auth (X-User-Id) replaces get_current_user dependency
- /tier/{user_id} endpoint for internal service-to-service lookups
- /features and /features/{tier} for feature matrix queries
- Dockerfile: single worker, 30s timeout (lightweight service)

services/billing/requirements.txt

@@ -0,0 +1,9 @@
+fastapi>=0.115.0
+uvicorn[standard]>=0.34.0
+gunicorn>=22.0.0
+pydantic>=2.10.0
+pydantic-settings>=2.7.0
+sqlalchemy>=2.0.0
+asyncpg>=0.30.0
+python-dotenv>=1.0.0
+stripe>=8.0.0