Deploy payments/dev dataplane (Layer 2), mirroring catalog/customers

Real Konnect Control Plane dc3b2a36-e2ad-4da1-a142-022d0ce6d94e, PAT
in Vault (kong-v2/payments/dev/konnect-pat). Replaces the TODO stubs
with the proven gateway-kustomization.yaml + externalsecret.yaml +
policies-kustomization.yaml pattern. IdP is the local Keycloak from
the start (platform default now, see STATUS.md 15/16), not the VPS.

Layer 3 (payments-api) not started — Gateway answers 404, nothing
routed yet.

dev/api-gateway/gateway-kustomization.yaml

@@ -0,0 +1,37 @@
+# sourceRef points at the GitRepository "k8s-platform" (created once,
+# flux-system) — any domain's Kustomization can reference this same
+# object, no monorepo: k8s-platform and dataplane-payments stay two
+# separate repos, `path` below is relative to the root of whichever repo
+# `sourceRef` names.
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: payments-dev-api-gateway
+  namespace: flux-system
+spec:
+  interval: 10m
+  sourceRef:
+    kind: GitRepository
+    name: k8s-platform
+  path: ./dataplane-template
+  prune: true
+  postBuild:
+    # Domain-specific non-sensitive values inline — no separate ConfigMap to
+    # forget to re-apply after an edit. Inline substitute always wins over
+    # substituteFrom regardless of order, so this still overrides
+    # dataplane-template-defaults where needed.
+    substitute:
+      NAME: payments-dev
+      NAMESPACE: payments
+      CONTROL_PLANE_ID: "dc3b2a36-e2ad-4da1-a142-022d0ce6d94e"
+      HOSTNAME: payments-dev.sttlab.local
+    # Cluster-wide defaults only — the Konnect PAT is NOT handled via
+    # substitute/substituteFrom at all: that mechanism requires the source
+    # Secret to sit in flux-system (same namespace as this Kustomization),
+    # which put a live copy of the real PAT in a shared, broadly-readable
+    # namespace. Rejected. The real konnect-auth Secret is materialized
+    # directly into the payments namespace instead (externalsecret.yaml, via
+    # External Secrets Operator + Vault) — never passes through flux-system.
+    substituteFrom:
+      - kind: ConfigMap
+        name: dataplane-template-defaults