Harden Companion deployment and release security

[MatchHar]

Summary

• keep Companion port 8083 off public and LAN interfaces in Docker Caddy deployments
• fail closed when DATABASE_PASS or the private Docker bridge gateway is unavailable
• pin runtime images by digest and clarify the supported-version/authentication boundary documentation
• add race, reachable-vulnerability, secret, CodeQL, and release-provenance checks

Validation

• go test ./...
• go vet ./...
• go test -race ./...
• go run golang.org/x/vuln/cmd/govulncheck@v1.1.4 ./...
• bash -n install.sh update.sh uninstall.sh backup.sh restore.sh storage-status.sh scripts/*.sh
• parsed all workflow YAML
• Gitleaks scan: no leaks found
• git diff --check

This is intentionally a draft until GitHub-hosted checks have completed.

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[MatchHar]

Superseded by the newer main branch security, HostBox, lock-secure, CI, and idempotent release-publishing work. Closing this conflicting draft so the current audited main branch is the single source of truth.