Add spool retention and inspection policies

[Its-donkey]

Summary

Introduce retention and management capabilities for stored messages under ./data/spool.

Motivation

Messages persist indefinitely and can accumulate without bounds. Providing configurable retention windows, size limits, or basic management APIs/tools will prevent unbounded growth and simplify reviewing stored artefacts.

Proposed direction

• Expose env vars or config files for retention duration, maximum total size, or per-recipient limits.
• Optionally add inspection or cleanup tooling (CLI or API) to list and purge stored messages.
• Ensure changes integrate cleanly with existing storage hashing and permissions.

Labels

• feature
• storage

[Its-donkey]

Implemented in PR #20 - Spool retention via SMTP_RETENTION_DAYS with automatic cleanup