fix(api): route tenants through origin gateway

docs/context/features/platform-vision/FACTS.jsonl

@@ -11,3 +11,4 @@
 {"id":"PV-20260818T104300Z-b3c4","subject":"RoutingContext","predicate":"is-required-on","object":"CheckoutSession, PaymentIntent, Payment, Refund and ReconciliationRecord; frozen at checkout-session creation and immutable thereafter, so a payment is always attributable to exactly one payment point","src":["docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md","docs/backend/PHASE-7-PAYMENTS-RECONCILIATION-CONTRACT.md"],"status":"active","kind":"constraint","updated_at":"2026-08-18T10:43:00Z","confidence":"high","tags":["payments","reconciliation","contract"]}
 {"id":"PV-20260818T104400Z-d9e2","subject":"partner-api-credentials","predicate":"are-scoped-by","object":"a single node whose subtree defines authority; we hold only the partner-generated public key, rotation runs on a bounded overlap window and revocation is immediate and irreversible","src":["docs/backend/PARTNER-PROVISIONING-API-CONTRACT.md","docs/backend/TRACK-S-SECURITY-RBAC-CONTRACT.md"],"status":"active","kind":"decision","updated_at":"2026-08-18T10:44:00Z","confidence":"high","tags":["security","credentials","partner"]}
 {"id":"PV-20260818T104500Z-a6f7","subject":"checkout-payment-methods","predicate":"already-support","object":"both qr and card end to end in src/app/pages/cart/cart.component.ts (separate create paths and separate status pollers); card is not an outstanding gap","src":["src/app/pages/cart/cart.component.ts","src/app/services/api.service.ts"],"status":"active","kind":"implemented","updated_at":"2026-08-18T10:45:00Z","confidence":"high","tags":["payments","frontend"]}
+{"id":"PV-20260820T095500Z-b17e","subject":"tenant-api-routing","predicate":"is-decided-to-use","object":"one same-origin {origin}/backend gateway for bootstrap, auth, legacy endpoints, and versioned /api endpoints; nginx strips /backend and preserves the original Host for server-side tenant resolution","src":["docs/context/adrs/ADR-0004-route-every-tenant-api-through-a-same-origin-gateway.md","src/app/core/config/api-config.service.ts","scripts/deploy/server-setup.sh"],"status":"active","kind":"decision","updated_at":"2026-08-20T09:55:00Z","confidence":"high","tags":["architecture","multi-tenant","api","routing","nginx"]}