Fixes CAR-161 UAT failure: k8s HTTPRoute forwards /api/* to the API
gateway without path rewriting, so requests arrive at FastAPI as
/api/v1/purchases, /api/v1/products, etc. FastAPI previously mounted
data routers at root, causing 404s on all /api/v1/* calls.
Keep health and auth routers at root (probes hit /health directly;
auth traffic is routed to the auth service via HTTPRoute).
Co-Authored-By: Paperclip <noreply@paperclip.ing>