fix: support sticky mail spools and pending recovery

[xxvcc]

Summary

• accept root-owned sticky world-writable mail pools such as Debian root:mail 3777 and Arch root:root 1777, while retaining fail-closed checks for unsafe roots and mailbox artifacts
• validate mail roots before useradd, return a captured identity for post-create failures, and make invite rollback/recovery durable
• add strictly interactive recovery for legacy pending rows and strengthen deletion/NSS/mail durability checks
• update Chinese and English operator, installation, security, release, README, and changelog contracts for v2.9.2

Validation

• go test -mod=readonly -count=1 -race ./...
• go vet -mod=readonly -printf.funcs=printf,errorf,warnf ./...
• staticcheck ./... with ordinary and integration tags
• govulncheck ./... (0 reachable vulnerabilities)
• go mod verify and go mod tidy -diff
• ShellCheck, shell syntax, actionlint, mirror receiver policy tests
• static amd64 and arm64 builds reporting version 2.9.2
• isolated Debian 13 root integration race suite
• Debian 13 root:mail 3777 invite/SSH/sudo/systemd/revoke canary
• Ubuntu 24.04 root:mail 2775 canary and unsafe 2777 pre-useradd refusal canary