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/service.yaml

@@ -0,0 +1,16 @@
+# TEMPLATE — cluster-internal service in front of the receiver pod.
+apiVersion: v1
+kind: Service
+metadata:
+  name: exploredns-receiver
+  namespace: exploredns-receiver
+  labels:
+    app: exploredns-receiver
+spec:
+  type: ClusterIP
+  selector:
+    app: exploredns-receiver
+  ports:
+    - name: http
+      port: 8080
+      targetPort: http