Add optional garbage collection services for registries

[obeone]

Summary

• allow registries to declare a garbageCollect schedule via cron or interval strings in the sample config
• generate a companion registry-gc service that periodically runs registry garbage-collect with the configured cadence
• document how to enable the automated garbage-collection service and how it complements TTL-based cache expiry

Testing

• pytest

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https://chatgpt.com/codex/tasks/task_e_68e845a6ef18832e944a6a4129802c7c