chore(receiver): packaging, k8s manifests, CI/release, docs

Dockerfile.receiver (CGO-free, /data volume), receiver image in CI and
tag releases, receiver binary in release archives, make build-receiver,
example k8s manifests (deployment/service/ingress/secret/pvc) under
deploy/k8s/receiver/, and README coverage including sender/receiver
token pairing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

deploy/k8s/receiver/ingress.yaml

@@ -0,0 +1,33 @@
+# TEMPLATE — replace receiver.example.com with your real host and wire up
+# TLS for your cluster (the webhook bearer token and admin password travel
+# in headers, so plain HTTP is not acceptable across the internet).
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: exploredns-receiver
+  namespace: exploredns-receiver
+  annotations: {}
+    # nginx ingress controller:
+    # cert-manager.io/cluster-issuer: letsencrypt
+    # nginx.ingress.kubernetes.io/proxy-body-size: 1m
+    #
+    # traefik:
+    # traefik.ingress.kubernetes.io/router.entrypoints: websecure
+    # traefik.ingress.kubernetes.io/router.tls: "true"
+spec:
+  # ingressClassName: nginx
+  tls:
+    - hosts:
+        - receiver.example.com
+      secretName: exploredns-receiver-tls # created by cert-manager or by hand
+  rules:
+    - host: receiver.example.com
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: exploredns-receiver
+                port:
+                  name: http