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>

.gitea/workflows/ci.yml

@@ -67,3 +67,13 @@ jobs:
           tags: |
             gitea.hansenits.com.au/hits/exploredns-web:latest
             gitea.hansenits.com.au/hits/exploredns-web:${{ github.sha }}
+
+      - name: Build and push receiver image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile.receiver
+          push: true
+          tags: |
+            gitea.hansenits.com.au/hits/exploredns-receiver:latest
+            gitea.hansenits.com.au/hits/exploredns-receiver:${{ github.sha }}