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/secret.yaml

@@ -0,0 +1,25 @@
+# TEMPLATE — fill in real values before applying, or create the secret
+# imperatively instead and never commit credentials:
+#
+#   kubectl -n exploredns-receiver create secret generic exploredns-receiver \
+#     --from-literal=RECEIVER_ADMIN_USER=admin \
+#     --from-literal=RECEIVER_ADMIN_PASSWORD='change-me' \
+#     --from-literal=RECEIVER_INGEST_TOKEN='change-me-too'
+#
+# The deployment loads every key here as an environment variable (envFrom).
+apiVersion: v1
+kind: Secret
+metadata:
+  name: exploredns-receiver
+  namespace: exploredns-receiver
+type: Opaque
+stringData:
+  RECEIVER_ADMIN_USER: admin
+  RECEIVER_ADMIN_PASSWORD: change-me
+  # Bearer token the main app must send on POST /webhook. Must match the
+  # sender's EXPLOREDNS_WEBHOOK_TOKEN. Leave unset to accept unauthenticated
+  # posts (not recommended for an internet-facing receiver).
+  RECEIVER_INGEST_TOKEN: change-me-too
+  # Uncomment to store events in an external MySQL instead of the SQLite
+  # file on the PVC (go-sql-driver DSN).
+  # RECEIVER_MYSQL_DSN: "user:pass@tcp(mysql.example.com:3306)/exploredns"